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

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.

The precise function of alphaherpesvirus tegument proteins and their interactions during the viral life cycle

Alphaherpesvirus is a widespread pathogen that causes diverse diseases in humans and animals and can severely damage host health. Alphaherpesvirus particles comprise a DNA core, capsid, tegument and envelope; the tegument is located between the nuclear capsid and envelope. According to biochemical and proteomic analyses of alphaherpesvirus particles, the tegument contains at least 24 viral proteins and plays an important role in the alphaherpesvirus life cycle. This article reviews the important role of tegument proteins and their interactions during the viral life cycle to provide a reference and inspiration for understanding alphaherpesvirus infection pathogenesis and identifying new antiviral strategies.

Leveraging biotin-based proximity labeling to identify cellular factors governing early alphaherpesvirus infection

Neurotropic alphaherpesviruses, including herpes simplex virus type 1 and pseudorabies virus, establish a lifelong presence within the peripheral nervous system of their mammalian hosts. Upon entering cells, two conserved tegument proteins, pUL36 and pUL37, traffic DNA-containing capsids to nuclei. These proteins support long-distance retrograde axonal transport and invasion of the nervous system in vivo. To better understand how pUL36 and pUL37 function, recombinant viral particles carrying BioID2 fused to these proteins were produced to biotinylate cellular proteins in their proximity (<10 nm) during infection. Eighty-six high-confidence host proteins were identified by mass spectrometry and subsequently targeted by CRISPR-Cas9 gene editing to assess their contributions to early infection. Proteins were identified that both supported and antagonized infection in immortalized human epithelial cells. The latter included zyxin, a protein that localizes to focal adhesions and regulates actin cytoskeletal dynamics. Zyxin knockout cells were hyper-permissive to infection and could be rescued with even modest expression of GFP-zyxin. These results provide a resource for studies of the virus-cell interface and identify zyxin as a novel deterrent to alphaherpesvirus infection.IMPORTANCENeuroinvasive alphaherpesviruses are highly prevalent with many members found across mammals [e.g., herpes simplex virus type 1 (HSV-1) in humans and pseudorabies virus in pigs]. HSV-1 causes a range of clinical manifestations from cold sores to blindness and encephalitis. There are no vaccines or curative therapies available for HSV-1. A fundamental feature of these viruses is their establishment of lifelong infection of the nervous system in their respective hosts. This outcome is possible due to a potent neuroinvasive property that is coordinated by two proteins: pUL36 and pUL37. In this study, we explore the cellular protein network in proximity to pUL36 and pUL37 during infection and examine the impact of knocking down the expression of these proteins upon infection.

COVID-19: Unveiling the Neuropsychiatric Maze-From Acute to Long-Term Manifestations

The SARS-CoV-2 virus has spread rapidly despite implementing strategies to reduce its transmission. The disease caused by this virus has been associated with a diverse range of symptoms, including common neurological manifestations such as dysgeusia, anosmia, and myalgias. Additionally, numerous cases of severe neurological complications associated with this disease have been reported, including encephalitis, stroke, seizures, and Guillain-Barré syndrome, among others. Given the high prevalence of neurological manifestations in this disease, the objective of this review is to analyze the mechanisms by which this virus can affect the nervous system, from its direct invasion to aberrant activation of the immune system and other mechanisms involved in the symptoms, including neuropsychiatric manifestations, to gain a better understanding of the disease and thus facilitate the search for effective therapeutic strategies.

The HSV-1 pUL37 protein promotes cell invasion by regulating the kinesin-1 motor

Neurotropic alphaherpesviruses, including herpes simplex virus type 1 (HSV-1), recruit microtubule motor proteins to invade cells. The incoming viral particle traffics to nuclei in a two-step process. First, the particle uses the dynein-dynactin motor to sustain transport to the centrosome. In neurons, this step is responsible for long-distance retrograde axonal transport and is an important component of the neuroinvasive property shared by these viruses. Second, a kinesin-dependent mechanism redirects the particle from the centrosome to the nucleus. We have reported that the kinesin motor used during the second step of invasion is assimilated into nascent virions during the previous round of infection. Here, we report that the HSV-1 pUL37 tegument protein suppresses the assimilated kinesin-1 motor during retrograde axonal transport. Region 2 (R2) of pUL37 was required for suppression and functioned independently of the autoinhibitory mechanism native to kinesin-1. Furthermore, the motor domain and proximal coiled coil of kinesin-1 were sufficient for HSV-1 assimilation, pUL37 suppression, and nuclear trafficking. pUL37 localized to the centrosome, the site of assimilated kinesin-1 activation during infection, when expressed in cells in the absence of other viral proteins; however, pUL37 did not suppress kinesin-1 in this context. These results indicate that the pUL37 tegument protein spatially and temporally regulates kinesin-1 via the amino-terminal motor region in the context of the incoming viral particle.

Impact of actin polymerization and filopodia formation on herpes simplex virus entry in epithelial, neuronal, and T lymphocyte cells

Herpes simplex virus type 1 (HSV-1) has been known as a common viral pathogen that can infect several parts of the body, leading to various clinical manifestations. According to this diverse manifestation, HSV-1 infection in many cell types was demonstrated. Besides the HSV-1 cell tropism, e.g., fibroblast, epithelial, mucosal cells, and neurons, HSV-1 infections can occur in human T lymphocyte cells, especially in activated T cells. In addition, several studies found that actin polymerization and filopodia formation support HSV-1 infection in diverse cell types. Hence, the goal of this review is to explore the mechanism of HSV-1 infection in various types of cells involving filopodia formation and highlight potential future directions for HSV-1 entry-related research. Moreover, this review covers several strategies for possible anti-HSV drugs focused on the entry step, offering insights into potential therapeutic interventions.

Cell Intrinsic Determinants of Alpha Herpesvirus Latency and Pathogenesis in the Nervous System

Alpha herpesvirus infections (α-HVs) are widespread, affecting more than 70% of the adult human population. Typically, the infections start in the mucosal epithelia, from which the viral particles invade the axons of the peripheral nervous system. In the nuclei of the peripheral ganglia, α-HVs establish a lifelong latency and eventually undergo multiple reactivation cycles. Upon reactivation, viral progeny can move into the nerves, back out toward the periphery where they entered the organism, or they can move toward the central nervous system (CNS). This latency-reactivation cycle is remarkably well controlled by the intricate actions of the intrinsic and innate immune responses of the host, and finely counteracted by the viral proteins in an effort to co-exist in the population. If this yin-yang- or Nash-equilibrium-like balance state is broken due to immune suppression or genetic mutations in the host response factors particularly in the CNS, or the presence of other pathogenic stimuli, α-HV reactivations might lead to life-threatening pathologies. In this review, we will summarize the molecular virus-host interactions starting from mucosal epithelia infections leading to the establishment of latency in the PNS and to possible CNS invasion by α-HVs, highlighting the pathologies associated with uncontrolled virus replication in the NS.

Comprehensive Analysis of the Tegument Proteins Involved in Capsid Transport and Virion Morphogenesis of Alpha, Beta and Gamma Herpesviruses

Herpesviruses are enveloped and have an amorphous protein layer surrounding the capsid, which is termed the tegument. Tegument proteins perform critical functions throughout the viral life cycle. This review provides a comprehensive and comparative analysis of the roles of specific tegument proteins in capsid transport and virion morphogenesis of selected, well-studied prototypes of each of the three subfamilies of Herpesviridae i.e., human herpesvirus-1/herpes simplex virus-1 (Alphaherpesvirinae), human herpesvirus-5/cytomegalovirus (Betaherpesvirinae) and human herpesvirus -8/Kaposi's sarcomavirus (Gammaherpesvirinae). Most of the current knowledge is based on alpha herpesviruses, in particular HSV-1. While some tegument proteins are released into the cytoplasm after virus entry, several tegument proteins remain associated with the capsid and are responsible for transport to and docking at the nucleus. After replication and capsid formation, the capsid is enveloped at the nuclear membrane, which is referred to as primary envelopment, followed by de-envelopment and release into the cytoplasm. This requires involvement of at least three tegument proteins. Subsequently, multiple interactions between tegument proteins and capsid proteins, other tegument proteins and glycoproteins are required for assembly of the virus particles and envelopment at the Golgi, with certain tegument proteins acting as the central hub for these interactions. Some redundancy in these interactions ensures appropriate morphogenesis.

The role of nuclear pores and importins for herpes simplex virus infection

Microtubule transport and nuclear import are functionally connected, and the nuclear pore complex (NPC) can interact with microtubule motors. For several alphaherpesvirus proteins, nuclear localization signals (NLSs) and their interactions with specific importin-α proteins have been characterized. Here, we review recent insights on the roles of microtubule motors, capsid-associated NLSs, and importin-α proteins for capsid transport, capsid docking to NPCs, and genome release into the nucleoplasm, as well as the role of importins for nuclear viral transcription, replication, capsid assembly, genome packaging, and nuclear capsid egress. Moreover, importin-α proteins exert antiviral effects by promoting the nuclear import of transcription factors inducing the expression of interferons (IFN), cytokines, and IFN-stimulated genes, and the IFN-inducible MxB restricts capsid docking to NPCs.

The Intersection of Innate Immune Pathways with the Latent Herpes Simplex Virus Genome

Innate immune responses can impact different stages of viral life cycles. Herpes simplex virus latent infection of neurons and subsequent reactivation provide a unique context for immune responses to intersect with different stages of infection. Here, we discuss recent findings linking neuronal innate immune pathways with the modulation of latent infection, acting at the time of reactivation and during initial neuronal infection to have a long-term impact on the ability of the virus to reactivate.

Role of Innate Interferon Responses at the Ocular Surface in Herpes Simplex Virus-1-Induced Herpetic Stromal Keratitis

Herpes simplex virus type 1 (HSV-1) is a highly successful pathogen that primarily infects epithelial cells of the orofacial mucosa. After initial lytic replication, HSV-1 enters sensory neurons and undergoes lifelong latency in the trigeminal ganglion (TG). Reactivation from latency occurs throughout the host’s life and is more common in people with a compromised immune system. HSV-1 causes various diseases depending on the site of lytic HSV-1 replication. These include herpes labialis, herpetic stromal keratitis (HSK), meningitis, and herpes simplex encephalitis (HSE). HSK is an immunopathological condition and is usually the consequence of HSV-1 reactivation, anterograde transport to the corneal surface, lytic replication in the epithelial cells, and activation of the host’s innate and adaptive immune responses in the cornea. HSV-1 is recognized by cell surface, endosomal, and cytoplasmic pattern recognition receptors (PRRs) and activates innate immune responses that include interferons (IFNs), chemokine and cytokine production, as well as the recruitment of inflammatory cells to the site of replication. In the cornea, HSV-1 replication promotes type I (IFN-α/β) and type III (IFN-λ) IFN production. This review summarizes our current understanding of HSV-1 recognition by PRRs and innate IFN-mediated antiviral immunity during HSV-1 infection of the cornea. We also discuss the immunopathogenesis of HSK, current HSK therapeutics and challenges, proposed experimental approaches, and benefits of promoting local IFN-λ responses.

Live Triple Gene-Deleted Pseudorabies Virus-Vectored Subunit PCV2b and CSFV Vaccine Undergoes an Abortive Replication Cycle in the TG Neurons following Latency Reactivation

Like other alpha herpesviruses, pseudorabies virus (PRV) establishes lifelong latency in trigeminal ganglionic (TG) neurons. Upon stress, the latent viruses in the TG neurons reactivate and are transported anterograde from the neuron cell bodies to the nerve endings in the nasal mucosa, where they replicate and are discharged in the nasal and oral secretions. Consequently, the virus is transmitted to other naïve animals. This cycle of latency and reactivation continues until the animal dies or is slaughtered. We have constructed a PRV triple mutant virus (PRVtmv) and used it as a live subunit vaccine vector against porcine circovirus 2b (PCV2b) and classical swine fever virus (CSFV) (PRVtmv+). We compared the latency reactivation properties of PRVtmv+ with its parent wild-type (wt) Becker strain following intranasal infection. The results showed that PRV wt and PRVtmv+ established latency in the TG neurons. Based on nasal virus shedding, immediate early (infected cell protein 0; ICP0) and late genes, MCP (major capsid protein) and gC (glycoprotein C) transcriptions, and viral DNA copy numbers in the TGs of latently infected and dexamethasone (Dex)-treated pigs, both PRV wt and PRVtmv+ reactivated from latency. We noticed that PRV wt virus replicated productively in the terminally differentiated, postmitotic TG neurons, but PRVtmv+ failed to replicate and, therefore, there was no virus production in the TG. In addition, we found that only the PRV wt virus was shed in the nasal secretions following the Dex-induced reactivation. Our results demonstrated that the PRVtmv+ is safe as a live viral subunit vaccine vector without the possibility of productive replication in the TG upon reactivation from latency and without subsequent nasal virus shedding. This property of PRVtmv+ precludes the possibility of vaccine virus circulation in pigs and the risk of reversion to virulence.

When herpes simplex virus encephalitis meets antiviral innate immunity

Herpes simplex virus (HSV) is the most common pathogen of infectious encephalitis, accounting for nearly half of the confirmed cases of encephalitis. Its clinical symptoms are often atypical. HSV PCR in cerebrospinal fluid is helpful for diagnosis, and the prognosis is usually satisfactory after regular antiviral treatment. Interestingly, some patients with recurrent encephalitis have little antiviral effect. HSV PCR in cerebrospinal fluid is negative, but glucocorticoid has a significant effect after treatment. Specific antibodies, such as the NMDA receptor antibody, the GABA receptor antibody, and even some unknown antibodies, can be isolated from cerebrospinal fluid, proving that the immune system contributes to recurrent encephalitis, but the specific mechanism is still unclear. Based on recent studies, we attempt to summarize the relationship between herpes simplex encephalitis and innate immunity, providing more clues for researchers to explore this field further.

Pseudorabies Virus Mutants Lacking US9 Are Defective in Cytoplasmic Assembly and Sorting of Virus Particles into Axons and Not in Axonal Transport

Herpes simplex virus (HSV) and varicella zoster virus (VZV) rely on transport of virus particles in neuronal axons to spread from sites of viral latency in sensory ganglia to peripheral tissues then on to other hosts. This process of anterograde axonal transport involves kinesin motors that move virus particles rapidly along microtubules. α-herpesvirus anterograde transport has been extensively studied by characterizing the porcine pseudorabies virus (PRV) and HSV, with most studies focused on two membrane proteins: gE/gI and US9. It was reported that PRV and HSV US9 proteins bind to kinesin motors, promoting tethering of virus particles on the motors, and furthering anterograde transport within axons. Alternatively, other models have argued that HSV and PRV US9 and gE/gI function in the cytoplasm and not in neuronal axons. Specifically, HSV gE/gI and US9 mutants are defective in the assembly of virus particles in the cytoplasm of neurons and the subsequent sorting of virus particles to cell surfaces and into axons. However, PRV US9 and gE/gI mutants have not been characterized for these cytoplasmic defects. We examined neurons infected with PRV mutants, one lacking both gE/gI and US9 and the other lacking just US9, by electron microscopy. Both PRV mutants exhibited similar defects in virus assembly and cytoplasmic sorting of virus particles to cell surfaces. As well, the mutants exhibited reduced quantities of infectious virus in neurons and in cell culture supernatants. We concluded that PRV US9 primarily functions in neurons to promote cytoplasmic steps in anterograde transport.

pUL36 Deubiquitinase Activity Augments Both the Initiation and the Progression of Lytic Herpes Simplex Virus Infection in IFN-Primed Cells

The evolutionarily conserved, structural HSV-1 tegument protein pUL36 is essential for both virus entry and assembly. While its N-terminal deubiquitinase (DUB) activity is dispensable for infection in cell culture, it is required for efficient virus spread in vivo, as it acts as a potent viral immune evasin. Interferon (IFN) induces the expression of hundreds of antiviral factors, including many ubiquitin modulators, which HSV-1 needs to neutralize to efficiently initiate a productive infection. Herein, we discover two functions of the conserved pUL36 DUB during lytic replication in cell culture in an understudied but equally important scenario of HSV-1 infection in IFN-treated cells. Our data indicate that the pUL36 DUB contributes to overcoming the IFN-mediated suppression of productive infection in both the early and late phases of HSV-1 infection. We show that incoming tegument-derived pUL36 DUB activity contributes to the IFN resistance of HSV-1 in IFN-primed cells to efficiently initiate lytic virus replication. Subsequently, the de novo expressed DUB augmented the efficiency of virus replication and increased the output of infectious virus. Notably, the DUB defect was only apparent when IFN was applied prior to infection. Our data indicate that IFN-induced defense mechanisms exist and that they work to both neutralize infectivity early on and slow the progression of HSV-1 replication in the late stages of infection. Also, our data indicate that pUL36 DUB activity contributes to the disarming of these host responses. IMPORTANCE HSV-1 is a ubiquitous human pathogen that is responsible for common cold sores and may also cause life-threatening disease. pUL36 is an essential, conserved herpesvirus protein with N-terminal deubiquitinating (DUB) activity. The DUB is dispensable for HSV-1 replication in cell culture but represents an important viral immune evasin in vivo. IFN plays a pivotal role in HSV-1 infection and suppresses viral replication both in vitro and in vivo. Here, we show that DUB activity contributes to overcoming IFN-induced cellular resistance in order to more efficiently initiate lytic replication and produce infectious virions. As such, DUB activity in the incoming virions increases their infectivity, while the de novo synthesized DUB augments productive infection. Thus, the HSV-1 DUB antagonizes the activity of IFN-inducible effector proteins to facilitate productive infection at multiple levels. Our findings underscore the importance of using more challenging cell culture systems to fully understand virus protein functions.

Theiler’s virus-induced demyelinating disease as an infectious model of progressive multiple sclerosis

Multiple sclerosis (MS) is a neuroinflammatory and neurodegenerative disease of unknown etiology. However, several studies suggest that infectious agents, e.g., Human Herpes Viruses (HHV), may be involved in triggering the disease. Molecular mimicry, bystander effect, and epitope spreading are three mechanisms that can initiate immunoreactivity leading to CNS autoimmunity in MS. Theiler’s murine encephalomyelitis virus (TMEV)-induced demyelinating disease (TMEV-IDD) is a pre-clinical model of MS in which intracerebral inoculation of TMEV results in a CNS autoimmune disease that causes demyelination, neuroaxonal damage, and progressive clinical disability. Given the spectra of different murine models used to study MS, this review highlights why TMEV-IDD represents a valuable tool for testing the viral hypotheses of MS. We initially describe how the main mechanisms of CNS autoimmunity have been identified across both MS and TMEV-IDD etiology. Next, we discuss how adaptive, innate, and CNS resident immune cells contribute to TMEV-IDD immunopathology and how this relates to MS. Lastly, we highlight the sexual dimorphism observed in TMEV-IDD and MS and how this may be tied to sexually dimorphic responses to viral infections. In summary, TMEV-IDD is an underutilized murine model that recapitulates many unique aspects of MS; as we learn more about the nature of viral infections in MS, TMEV-IDD will be critical in testing the future therapeutics that aim to intervene with disease onset and progression.

Resveratrol Inhibits Pseudorabies Virus Replication by Targeting IE180 Protein

Resveratrol is a natural polyphenolic product in red wine and peanuts and has many pharmacological activities in humans. Our previous studies showed that resveratrol has good antiviral activity against the pseudorabies virus (PRV). However, little is known about the antiviral mechanism of resveratrol against PRV. In this study, we found that resveratrol inhibited the nuclear localization of IE180 protein, which is an important step for activating early/late genes transcription. Interestingly, the results show that resveratrol inhibited the activity of IE180 protein by dual-luciferase assay. Furthermore, molecular docking analysis shows that resveratrol could bind to the Thr601, Ser603, and Pro606 of IE180 protein. Point mutation assay confirmed that resveratrol lost its inhibition activity against the mutant IE180 protein. The results demonstrate that resveratrol exerts its antiviral activity against PRV by targeting the Thr601/Ser603/Pro606 sites of IE180 protein and inhibiting the transcriptional activation activity of IE180 protein. This study provides a novel insight into the antiviral mechanism of resveratrol against herpes viruses.

Bovine Herpesvirus 1 Invasion of Sensory Neurons by Retrograde Axonal Transport Is Dependent on the pUL37 Region 2 Effector

Following exposure and replication at mucosal surfaces, most alphaherpesviruses invade the peripheral nervous system by retrograde axonal transport and establish lifelong latent infections in the peripheral ganglia. Reactivation of ganglionic infections is followed by anterograde axonal transport of virions back to body surfaces where viral replication results in disease that can range from moderate to severe in presentation. In the case of bovine herpesvirus 1 (BoHV-1), replication in the epithelial mucosa presents as infectious bovine rhinotracheitis (IBR), a respiratory disease of significant economic impact. In this study, we provide a live-cell analysis of BoHV-1 retrograde axonal transport relative to the model alphaherpesvirus pathogen pseudorabies virus (PRV) and demonstrate that this critical neuroinvasive step is conserved between the two viruses. In addition, we report that the BoHV-1 pUL37 tegument protein supports processive retrograde motion in infected axons and invasion of the calf peripheral nervous system. IMPORTANCE A molecular and cellular understanding of the retrograde axonal transport process that underlies the neuroinvasive properties of the alphaherpesviruses is established from studies of herpes simplex virus and pseudorabies virus. The degree to which this phenotype is conserved in other related viruses has largely not been examined. We provide a time-lapse analysis of the retrograde axonal transport kinetics of bovine herpesvirus 1 and demonstrate that mutation of the pUL37 region 2 effector affords a strategy to produce live-attenuated vaccines for enhanced protection of cattle.

Microglia in antiviral immunity of the brain and spinal cord

Viral infections of the central nervous system (CNS) are a significant cause of neurological impairment and mortality worldwide. As tissue resident macrophages, microglia are critical initial responders to CNS viral infection. Microglia seem to coordinate brain-wide antiviral responses of both brain resident cells and infiltrating immune cells. This review discusses how microglia may promote this antiviral response at a molecular level, from potential mechanisms of virus recognition to downstream cytokine responses and interaction with antiviral T cells. Recent advancements in genetic tools to specifically target microglia in vivo promise to further our understanding about the precise mechanistic role of microglia in CNS infection.

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

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

Innate Immune Signaling and Role of Glial Cells in Herpes Simplex Virus- and Rabies Virus-Induced Encephalitis

The environment of the central nervous system (CNS) represents a double-edged sword in the context of viral infections. On the one hand, the infectious route for viral pathogens is restricted via neuroprotective barriers; on the other hand, viruses benefit from the immunologically quiescent neural environment after CNS entry. Both the herpes simplex virus (HSV) and the rabies virus (RABV) bypass the neuroprotective blood-brain barrier (BBB) and successfully enter the CNS parenchyma via nerve endings. Despite the differences in the molecular nature of both viruses, each virus uses retrograde transport along peripheral nerves to reach the human CNS. Once inside the CNS parenchyma, HSV infection results in severe acute inflammation, necrosis, and hemorrhaging, while RABV preserves the intact neuronal network by inhibiting apoptosis and limiting inflammation. During RABV neuroinvasion, surveilling glial cells fail to generate a sufficient type I interferon (IFN) response, enabling RABV to replicate undetected, ultimately leading to its fatal outcome. To date, we do not fully understand the molecular mechanisms underlying the activation or suppression of the host inflammatory responses of surveilling glial cells, which present important pathways shaping viral pathogenesis and clinical outcome in viral encephalitis. Here, we compare the innate immune responses of glial cells in RABV- and HSV-infected CNS, highlighting different viral strategies of neuroprotection or Neuroinflamm. in the context of viral encephalitis.

Herpesviruses assimilate kinesin to produce motorized viral particles

Neurotropic alphaherpesviruses initiate infection in exposed mucosal tissues and, unlike most viruses, spread rapidly to sensory and autonomic nerves where life-long latency is established¹. Recurrent infections arise sporadically from the peripheral nervous system throughout the life of the host, and invasion of the central nervous system may occur, with severe outcomes². These viruses directly recruit cellular motors for transport along microtubules in nerve axons, but how the motors are manipulated to deliver the virus to neuronal nuclei is not understood. Here, using herpes simplex virus type I and pseudorabies virus as model alphaherpesviruses, we show that a cellular kinesin motor is captured by virions in epithelial cells, carried between cells, and subsequently used in neurons to traffic to nuclei. Viruses assembled in the absence of kinesin are not neuroinvasive. The findings explain a critical component of the alphaherpesvirus neuroinvasive mechanism and demonstrate that these viruses assimilate a cellular protein as an essential proviral structural component. This principle of viral assimilation may prove relevant to other virus families and offers new strategies to combat infection.

The Hippocampal Vulnerability to Herpes Simplex Virus Type I Infection: Relevance to Alzheimer's Disease and Memory Impairment

Herpes simplex virus type 1 (HSV-1) as a possible infectious etiology in Alzheimer’s disease (AD) has been proposed since the 1980s. The accumulating research thus far continues to support the association and a possible causal role of HSV-1 in the development of AD. HSV-1 has been shown to induce neuropathological and behavioral changes of AD, such as amyloid-beta accumulation, tau hyperphosphorylation, as well as memory and learning impairments in experimental settings. However, a neuroanatomical standpoint of HSV-1 tropism in the brain has not been emphasized in detail. In this review, we propose that the hippocampal vulnerability to HSV-1 infection plays a part in the development of AD and amnestic mild cognitive impairment (aMCI). Henceforth, this review draws on human studies to bridge HSV-1 to hippocampal-related brain disorders, namely AD and aMCI/MCI. Next, experimental models and clinical observations supporting the neurotropism or predilection of HSV-1 to infect the hippocampus are examined. Following this, factors and mechanisms predisposing the hippocampus to HSV-1 infection are discussed. In brief, the hippocampus has high levels of viral cellular receptors, neural stem or progenitor cells (NSCs/NPCs), glucocorticoid receptors (GRs) and amyloid precursor protein (APP) that support HSV-1 infectivity, as well as inadequate antiviral immunity against HSV-1. Currently, the established diseases HSV-1 causes are mucocutaneous lesions and encephalitis; however, this review revises that HSV-1 may also induce and/or contribute to hippocampal-related brain disorders, especially AD and aMCI/MCI.

The journey of herpesvirus capsids and genomes to the host cell nucleus

Starting a herpesviral infection is a steeplechase across membranes, cytosol, and nuclear envelopes and against antiviral defence mechanisms. Here, we highlight recent insights on capsid stabilization at the portals during assembly, early capsid-host interactions ensuring nuclear targeting of incoming capsids, and genome uncoating. After fusion with a host membrane, incoming capsids recruit microtubule motors for traveling to the centrosome, and by unknown mechanisms get forward towards the nucleus. The interaction of capsid-associated tegument proteins with nucleoporins orients the capsid portal towards the nuclear pore, and presumably after removal of the portal caps the genomes that have been packaged under pressure can be injected into the nucleoplasm for transcription and replication. Some cell types disarm the incoming capsids or silence the incoming genomes to reduce the likelihood of infection.

The Influence of Virus Infection on Microglia and Accelerated Brain Aging

Microglia are the resident immune cells of the central nervous system contributing substantially to health and disease. There is increasing evidence that inflammatory microglia may induce or accelerate brain aging, by interfering with physiological repair and remodeling processes. Many viral infections affect the brain and interfere with microglia functions, including human immune deficiency virus, flaviviruses, SARS-CoV-2, influenza, and human herpes viruses. Especially chronic viral infections causing low-grade neuroinflammation may contribute to brain aging. This review elucidates the potential role of various neurotropic viruses in microglia-driven neurocognitive deficiencies and possibly accelerated brain aging.

Engagement of Neurotropic Viruses in Fast Axonal Transport: Mechanisms, Potential Role of Host Kinases and Implications for Neuronal Dysfunction

Much remains unknown about mechanisms sustaining the various stages in the life cycle of neurotropic viruses. An understanding of those mechanisms operating before their replication and propagation could advance the development of effective anti-viral strategies. Here, we review our current knowledge of strategies used by neurotropic viruses to undergo bidirectional movement along axons. We discuss how the invasion strategies used by specific viruses might influence their mode of interaction with selected components of the host’s fast axonal transport (FAT) machinery, including specialized membrane-bounded organelles and microtubule-based motor proteins. As part of this discussion, we provide a critical evaluation of various reported interactions among viral and motor proteins and highlight limitations of some in vitro approaches that led to their identification. Based on a large body of evidence documenting activation of host kinases by neurotropic viruses, and on recent work revealing regulation of FAT through phosphorylation-based mechanisms, we posit a potential role of host kinases on the engagement of viruses in retrograde FAT. Finally, we briefly describe recent evidence linking aberrant activation of kinase pathways to deficits in FAT and neuronal degeneration in the context of human neurodegenerative diseases. Based on these findings, we speculate that neurotoxicity elicited by viral infection may involve deregulation of host kinases involved in the regulation of FAT and other cellular processes sustaining neuronal function and survival.

Understanding the Role of Blood Vessels in the Neurologic Manifestations of Coronavirus Disease 2019 (COVID-19)

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was originally identified as an outbreak in Wuhan, China, toward the end of 2019 and quickly became a global pandemic, with a large death toll. Originally identified as a respiratory disease, similar to previously discovered SARS and Middle East respiratory syndrome (MERS), concern has since been raised about the effects of SARS-CoV-2 infection on the vasculature. This viral-vascular involvement is of particular concern with regards to the small vessels present in the brain, with mounting evidence demonstrating that SARS-CoV-2 is capable of crossing the blood-brain barrier. Severe symptoms, termed coronavirus disease 2019 (COVID-19), often result in neurologic complications, regardless of patient age. These neurologic complications range from mild to severe across all demographics; however, the long-term repercussions of neurologic involvement on patient health are still unknown.

Crosstalk Between Epithelial Cells, Neurons and Immune Mediators in HSV-1 Skin Infection

Herpes simplex virus type 1 (HSV-1) infection is highly prevalent in humans, with approximately two-thirds of the world population living with this virus. However, only a fraction of those carrying HSV-1, which elicits lifelong infections, are symptomatic. HSV-1 mainly causes lesions in the skin and mucosae but reaches the termini of sensory neurons innervating these tissues and travels in a retrograde manner to the neuron cell body where it establishes persistent infection and remains in a latent state until reactivated by different stimuli. When productive reactivations occur, the virus travels back along axons to the primary infection site, where new rounds of replication are initiated in the skin, in recurrent or secondary infections. During this process, new neuron infections occur. Noteworthy, the mechanisms underlying viral reactivations and the exit of latency are somewhat poorly understood and may be regulated by a crosstalk between the infected neurons and components of the immune system. Here, we review and discuss the immune responses that occur at the skin during primary and recurrent infections by HSV-1, as well as at the interphase of latently-infected neurons. Moreover, we discuss the implications of neuronal signals over the priming and migration of immune cells in the context of HSV-1 infection.

Retrograde Axonal Transport Property of Adeno-Associated Virus and Its Possible Application in Future

Gene therapy has become a treatment method for many diseases. Adeno-associated virus (AAV) is one of the most common virus vectors, is also widely used in the gene therapy field. During the past 2 decades, the retrograde axonal transportability of AAV has been discovered and utilized. Many studies have worked on the retrograde axonal transportability of AAV, and more and more people are interested in this field. This review described the current application, influence factors, and mechanism of retrograde axonal transportability of AAV and predicted its potential use in disease treatment in near future.

Cellular and Viral Determinants of HSV-1 Entry and Intracellular Transport towards Nucleus of Infected Cells

HSV-1 employs cellular motor proteins and modulates kinase pathways to facilitate intracellular virion capsid transport. Previously, we and others have shown that the Akt inhibitor miltefosine inhibited virus entry. Herein, we show that the protein kinase C inhibitors staurosporine (STS) and gouml inhibited HSV-1 entry into Vero cells, and that miltefosine prevents HSV-1 capsid transport toward the nucleus. We have reported that the HSV-1 UL37 tegument protein interacts with the dynein motor complex during virus entry and virion egress, while others have shown that the UL37/UL36 protein complex binds dynein and kinesin causing a saltatory movement of capsids in neuronal axons. Co-immoprecipitation experiments confirmed previous findings from our laboratory that the UL37 protein interacted with the dynein intermediate chain (DIC) at early times post infection. This UL37-DIC interaction was concurrent with DIC phosphorylation in infected, but not mock-infected cells. Miltefosine inhibited dynein phosphorylation when added before, but not after virus entry. Inhibition of motor accessory protein dynactins (DCTN2, DCTN3), the adaptor proteins EB1 and the Bicaudal D homolog 2 (BICD2) expression using lentiviruses expressing specific shRNAs, inhibited intracellular transport of virion capsids toward the nucleus of human neuroblastoma (SK-N-SH) cells. Co-immunoprecipitation experiments revealed that the major capsid protein Vp5 interacted with dynactins (DCTN1/p150 and DCTN4/p62) and the end-binding protein (EB1) at early times post infection. These results show that Akt and kinase C are involved in virus entry and intracellular transport of virion capsids, but not in dynein activation via phosphorylation. Importantly, both the UL37 and Vp5 viral proteins are involved in dynein-dependent transport of virion capsids to the nuclei of infected cells.Importance. Herpes simplex virus type-1 enter either via fusion at the plasma membranes or endocytosis depositing the virion capsids into the cytoplasm of infected cells. The viral capsids utilize the dynein motor complex to move toward the nuclei of infected cells using the microtubular network. This work shows that inhibitors of the Akt kinase and kinase C inhibit not only viral entry into cells but also virion capsid transport toward the nucleus. In addition, the work reveals that the virion protein ICP5 (VP5) interacts with the dynein cofactor dynactin, while the UL37 protein interacts with the dynein intermediate chain (DIC). Importantly, neither Akt nor Kinase C was found to be responsible for phosphorylation/activation of dynein indicating that other cellular or viral kinases may be involved.

Anterograde transport of α-herpesviruses in neuronal axons

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

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.

Global prevalence and pathogenesis of headache in COVID-19: A systematic review and meta-analysis

Background: This study was conducted to determine the prevalence of headache in coronavirus disease 2019 (COVID-19) and to assess its association as a predictor for COVID-19. This study also aimed to discuss the possible pathogenesis of headache in COVID-19. Methods: Available articles from PubMed, Scopus, and Web of Science were searched as of September 2 nd, 2020. Data on characteristics of the study, headache and COVID-19 were extracted following the PRISMA guidelines. Biases were assessed using the Newcastle-Ottawa scale. The cumulative prevalence of headache was calculated for the general population (i.e. adults and children). The pooled odd ratio (OR) with 95% confidence intervals (95%CI) was calculated using the Z test to assess the association between headache and the presence of COVID-19 cases. Results: We included 104,751 COVID-19 cases from 78 eligible studies to calculate the global prevalence of headache in COVID-19 and 17 studies were included to calculate the association of headache and COVID-19. The cumulative prevalence of headache in COVID-19 was 25.2% (26,464 out of 104,751 cases). Headache was found to be more prevalent, approximately by two-fold, in COVID-19 patients than in non-COVID-19 patients with symptoms of other respiratory viral infections, OR: 1.73; 95% CI: 1.94, 2.5 with p=0.04. Conclusion: Headache is common among COVID-19 patients and seems to be more common in COVID-19 patients compared to those with the non-COVID-19 viral infection. No definitive mechanisms on how headache  emerges in COVID-19 patients but several possible hypotheses have been proposed. However, extensive studies are warranted to elucidate the mechanisms. PROSPERO registration: CRD42020210332 (28/09/2020).

Global prevalence and pathogenesis of headache in COVID-19: A systematic review and meta-analysis

Background: This study was conducted to determine the prevalence of headache in coronavirus disease 2019 (COVID-19) and to assess its association as a predictor for COVID-19. This study also aimed to discuss the possible pathogenesis of headache in COVID-19. Methods: Available articles from PubMed, Scopus, and Web of Science were searched as of September 2 nd, 2020. Data on characteristics of the study, headache and COVID-19 were extracted following the PRISMA guidelines. Biases were assessed using the Newcastle-Ottawa scale. The cumulative prevalence of headache was calculated for the general population (i.e. adults and children). The pooled odd ratio (OR) with 95% confidence intervals (95%CI) was calculated using the Z test to assess the association between headache and the presence of COVID-19 cases. Results: We included 104,751 COVID-19 cases from 78 eligible studies to calculate the global prevalence of headache in COVID-19 and 17 studies were included to calculate the association of headache and COVID-19. The cumulative prevalence of headache in COVID-19 was 25.2% (26,464 out of 104,751 cases). Headache was found to be more prevalent, approximately by two-fold, in COVID-19 patients than in non-COVID-19 patients (other respiratory viral infections), OR: 1.73; 95% CI: 1.94, 2.5 with p=0.04. Conclusion: Headache is common among COVID-19 patients and seems to be more common in COVID-19 patients compared to those with the non-COVID-19 viral infection. No definitive mechanisms on how headache  emerges in COVID-19 patients but several possible hypotheses have been proposed. However, extensive studies are warranted to elucidate the mechanisms. PROSPERO registration: CRD42020210332 (28/09/2020).

Characterization of the Herpes Simplex Virus (HSV) Tegument Proteins That Bind to gE/gI and US9, Which Promote Assembly of HSV and Transport into Neuronal Axons

The herpes simplex virus (HSV) heterodimer gE/gI and another membrane protein, US9, which has neuron-specific effects, promote the anterograde transport of virus particles in neuronal axons. Deletion of both HSV gE and US9 blocks the assembly of enveloped particles in the neuronal cytoplasm, which explains why HSV virions do not enter axons. Cytoplasmic envelopment depends upon interactions between viral membrane proteins and tegument proteins that encrust capsids. We report that tegument protein UL16 is unstable, i.e., rapidly degraded, in neurons infected with a gE-/US9- double mutant. Immunoprecipitation experiments with lysates of HSV-infected neurons showed that UL16 and three other tegument proteins, namely, VP22, UL11, and UL21, bound either to gE or gI. All four of these tegument proteins were also pulled down with US9. In neurons transfected with tegument proteins and gE/gI or US9, there was good evidence that VP22 and UL16 bound directly to US9 and gE/gI. However, there were lower quantities of these tegument proteins that coprecipitated with gE/gI and US9 from transfected cells than those of infected cells. This apparently relates to a matrix of several different tegument proteins formed in infected cells that bind to gE/gI and US9. In cells transfected with individual tegument proteins, this matrix is less prevalent. Similarly, coprecipitation of gE/gI and US9 was observed in HSV-infected cells but not in transfected cells, which argued against direct US9-gE/gI interactions. These studies suggest that gE/gI and US9 binding to these tegument proteins has neuron-specific effects on virus HSV assembly, a process required for axonal transport of enveloped particles.IMPORTANCE Herpes simplex viruses 1 and 2 and varicella-zoster virus cause significant morbidity and mortality. One basic property of these viruses is the capacity to establish latency in the sensory neurons and to reactivate from latency and then cause disease in peripheral tissues, such as skin and mucosal epithelia. The transport of nascent HSV particles from neuron cell bodies into axons and along axons to axon tips in the periphery is an important component of this reactivation and reinfection. Two HSV membrane proteins, gE/gI and US9, play an essential role in these processes. Our studies help elucidate how HSV gE/gI and US9 promote the assembly of virus particles and sorting of these virions into neuronal axons.

Superinfection and recombination of infectious laryngotracheitis virus vaccines in the natural host

Infectious laryngotracheitis virus (ILTV, Gallid alphaherpesvirus 1) causes severe respiratory disease in chickens and has a major impact on the poultry industry worldwide. Live attenuated vaccines are widely available and are administered early in the life of commercial birds, often followed by one or more rounds of revaccination, generating conditions that can favour recombination between vaccines. Better understanding of the factors that contribute to the generation of recombinant ILTVs will inform the safer use of live attenuated herpesvirus vaccines. This study aimed to examine the parameters of infection that allow superinfection and may enable the generation of recombinant progeny in the natural host. In this study, 120 specific-pathogen free (SPF) chickens in 8 groups were inoculated with two genetically distinct live-attenuated ILTV vaccine strains with 1-4 days interval between the first and second vaccinations. After inoculation, viral genomes were detected in tracheal swabs in all groups, with lowest copies detected in swabs collected from the groups where the interval between inoculations was 4 days. Superinfection of the host was defined as the detection of the virus that was inoculated last, and this was detected in tracheal swabs from all groups. Virus could be isolated from swabs at a limited number of timepoints, and these further illustrated superinfection of the birds as recombinant viruses were detected among the progeny. This study has demonstrated superinfection at host level and shows recombination events occur under a very broad range of infection conditions. The occurrence of superinfection after unsynchronised infection with multiple viruses, and subsequent genomic recombination, highlight the importance of using only one type of vaccine per flock as the most effective way to limit recombination.

Control of Orienting Movements and Locomotion by Projection-Defined Subsets of Brainstem V2a Neurons

Spatial orientation requires the execution of lateralized movements and a change in the animal's heading in response to multiple sensory modalities. While much research has focused on the circuits for sensory integration, chiefly to the midbrain superior colliculus (SC), the downstream cells and circuits that engage adequate motor actions have remained elusive. Furthermore, the mechanisms supporting trajectory changes are still speculative. Here, using transneuronal viral tracings in mice, we show that brainstem V2a neurons, a genetically defined subtype of glutamatergic neurons of the reticular formation, receive putative synaptic inputs from the contralateral SC. This makes them a candidate relay of lateralized orienting commands. We next show that unilateral optogenetic activations of brainstem V2a neurons in vivo evoked ipsilateral orienting-like responses of the head and the nose tip on stationary mice. When animals are walking, similar stimulations impose a transient locomotor arrest followed by a change of trajectory. Third, we reveal that these distinct motor actions are controlled by dedicated V2a subsets each projecting to a specific spinal cord segment, with at least (1) a lumbar-projecting subset whose unilateral activation specifically controls locomotor speed but neither impacts trajectory nor evokes orienting movements, and (2) a cervical-projecting subset dedicated to head orientation, but not to locomotor speed. Activating the latter subset suffices to steer the animals' directional heading, placing the head orientation as the prime driver of locomotor trajectory. V2a neurons and their modular organization may therefore underlie the orchestration of multiple motor actions during multi-faceted orienting behaviors.

Herpes Simplex Virus 2 Counteracts Neurite Outgrowth Repulsion during Infection in a Nerve Growth Factor-Dependent Manner

Herpes simplex virus 2 (HSV-2) is a prevalent human pathogen that establishes lifelong latency in neurons of the peripheral nervous system. Colonization of neurons is required for HSV-2 persistence and pathogenesis. The viral and cellular factors required for efficient infection of neurons are not fully understood. We show here that nonneuronal cells repel neurite outgrowth of sensory neurons, while HSV-2 infection overcomes this inhibition and, rather, stimulates neurite outgrowth. HSV-2 glycoprotein G and nerve growth factor contribute to this phenotype, which may attract neurites to sites of infection and facilitate virus spread to neurons. Understanding the mechanisms that modulate neurite outgrowth and facilitate HSV-2 infection of neurons might foster the development of therapeutics to reduce HSV-2 colonization of the nervous system and provide insights on neurite outgrowth and regeneration.

During primary infection, herpes simplex virus 2 (HSV-2) replicates in epithelial cells and enters neurites to infect neurons of the peripheral nervous system. Growth factors and attractive and repulsive directional cues influence neurite outgrowth and neuronal survival. We hypothesized that HSV-2 modulates the activity of such cues to increase neurite outgrowth. To test this hypothesis, we exposed sensory neurons to nerve growth factor (NGF) and mock- or HSV-2-infected HEK-293T cells, since they express repellents of neurite outgrowth. We show that HEK-293T cells secrete factors that inhibit neurite outgrowth, while infection with HSV-2 strains MS and 333 reduces this repelling phenotype, increasing neurite numbers. The HSV-2-mediated restoration of neurite outgrowth required the activity of NGF. In the absence of infection, however, NGF did not overcome the repulsion mediated by HEK-293T cells. We previously showed that recombinant, soluble glycoprotein G of HSV-2 (rSgG2) binds and enhances NGF activity, increasing neurite outgrowth. However, the effect of gG2 during infection has not been investigated. Therefore, we addressed whether gG2 contributes to overcoming neurite outgrowth repulsion. To do so, we generated viruses lacking gG2 expression and complemented them by exogenous expression of gG2. Overall, our results suggest that HSV-2 infection of nonneuronal cells reduces their repelling effect on neurite outgrowth in an NGF-dependent manner. gG2 contributed to this phenotype, but it was not the only factor. The enhanced neurite outgrowth may facilitate HSV-2 spread from epithelial cells into neurons expressing NGF receptors and increase HSV-2-mediated pathogenesis.

IMPORTANCE Herpes simplex virus 2 (HSV-2) is a prevalent human pathogen that establishes lifelong latency in neurons of the peripheral nervous system. Colonization of neurons is required for HSV-2 persistence and pathogenesis. The viral and cellular factors required for efficient infection of neurons are not fully understood. We show here that nonneuronal cells repel neurite outgrowth of sensory neurons, while HSV-2 infection overcomes this inhibition and, rather, stimulates neurite outgrowth. HSV-2 glycoprotein G and nerve growth factor contribute to this phenotype, which may attract neurites to sites of infection and facilitate virus spread to neurons. Understanding the mechanisms that modulate neurite outgrowth and facilitate HSV-2 infection of neurons might foster the development of therapeutics to reduce HSV-2 colonization of the nervous system and provide insights on neurite outgrowth and regeneration.

Cryo-EM structure of the varicella-zoster virus A-capsid

Varicella-zoster virus (VZV), a member of the Alphaherpesvirinae subfamily, causes severe diseases in humans of all ages. The viral capsids play critical roles in herpesvirus infection, making them potential antiviral targets. Here, we present the 3.7-Å-resolution structure of the VZV A-capsid and define the molecular determinants underpinning the assembly of this complicated viral machinery. Overall, the VZV capsid has a similar architecture to that of other known herpesviruses. The major capsid protein (MCP) assembles into pentons and hexons, forming extensive intra- and inter-capsomer interaction networks that are further secured by the small capsid protein (SCP) and the heterotriplex. The structure reveals a pocket beneath the floor of MCP that could potentially be targeted by antiviral inhibitors. In addition, we identified two alphaherpesvirus-specific structural features in SCP and Tri1 proteins. These observations highlight the divergence of different herpesviruses and provide an important basis for developing antiviral drugs.

Varicella-zoster virus (VZV) is the causative agent of chickenpox and herpes zoster (shingles). Cryo-EM structure of VZV capsid provides insights into the capsid assembly and reveals a pocket that could potentially be targeted by antiviral drugs.

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.

How Close are We to a COVID-19 Vaccine?

This review provides an overview regarding the main aspects of candidate COVID-19 vaccines and pathophysiology of disease. The types of biotechnological candidate vaccines to be developed against COVID-19, their degree of protection and the pathophysiological mechanism of the disease were analyzed in this review article. The literature data on which cruxes for the development of biotechnological candidate vaccines to be wended are based was researched. Data that could give reference to various biotechnological candidate vaccines were reviewed. For this purpose, up-to-date literature data was utilized. The ways to succeed in the development of a vaccine requiring a technological infrastructure are to synthesize the data obtained from long term trials and to put them into practice subsequently. The vaccines to be developed by means of recombinant DNA technology will be a source of inspiration to people for further studies. After a rapid process of vaccine development, the use of COVID-19 vaccine can be mainstreamed among people to prevent the disease. As a result of these practices, the evaluation of which vaccine will be more safe, reliable and effective will be performed after phase studies.

Mechanisms of Pathogen Invasion into the Central Nervous System

CNS infections continue to rise in incidence in conjunction with increases in immunocompromised populations or conditions that contribute to the emergence of pathogens, such as global travel, climate change, and human encroachment on animal territories. The severity and complexity of these diseases is impacted by the diversity of etiologic agents and their routes of neuroinvasion. In this review, we present historical, clinical, and molecular concepts regarding the mechanisms of pathogen invasion of the CNS. We also discuss the structural components of CNS compartments that influence pathogen entry and recent discoveries of the pathways exploited by pathogens to facilitate CNS infections. Advances in our understanding of the CNS invasion mechanisms of different neurotropic pathogens may enable the development of strategies to control their entry and deliver drugs to mitigate established infections.

Reovirus uses macropinocytosis-mediated entry and fast axonal transport to infect neurons

Several barriers protect the central nervous system (CNS) from pathogen invasion. Yet viral infections of the CNS are common and often debilitating. Understanding how neurotropic viruses co-opt host machinery to overcome challenges to neuronal entry and transmission is important to combat these infections. Neurotropic reovirus disseminates through neural routes and invades the CNS to cause lethal encephalitis in newborn animals. To define mechanisms of reovirus neuronal entry and directional transport, we used primary neuron cultures, which reproduce in vivo infection patterns displayed by different reovirus serotypes. Treatment of neurons with small-molecule inhibitors of different endocytic uptake pathways allowed us to discover that the cellular machinery mediating macropinocytosis is required for reovirus neuronal entry. This mechanism of reovirus entry differs from clathrin-mediated endocytosis, which is used by reovirus to invade non-neuronal cells. Analysis of reovirus transport and release from isolated soma or axonal termini of neurons cultivated in microfluidic devices indicates that reovirus is capable of retrograde but only limited anterograde neuronal transmission. The dynamics of retrograde reovirus movement are consistent with fast axonal transport coordinated by dynein along microtubules. Further analysis of viral transport revealed that multiple virions are transported together in axons within non-acidified vesicles. Reovirus-containing vesicles acidify after reaching the soma, where disassembly of virions and release of the viral core into the cytoplasm initiates replication. These results define mechanisms of reovirus neuronal entry and transport and establish a foundation to identify common host factors used by neuroinvasive viruses. Furthermore, our findings emphasize consideration of cell type-specific entry mechanisms in the tailored design of neurotropic viruses as tracers, oncolytic agents, and delivery vectors.

Viral infections of the central nervous system (CNS) cause a significant health burden globally and compel a better mechanistic understanding of neural invasion by viruses to develop effective interventions. Neurotropic reovirus disseminates through neural routes to infect the CNS and serves as a tractable model to study neural invasion by viruses. Despite knowledge of reovirus neurotropism for decades, mechanisms mediating reovirus neuronal infection remain undefined. We used primary neurons cultured in microfluidic devices to study entry and directional transport of reovirus. We discovered that reovirus uses macropinocytosis for neuronal entry as opposed to the use of a clathrin-mediated pathway in non-neuronal cells. We are unaware of another virus using macropinocytosis to enter neurons. Following internalization, reovirus spreads in the retrograde direction using dynein-mediated fast axonal transport but exhibits limited anterograde spread. We further demonstrate that reovirus disassembly and replication occur in the neuronal soma subsequent to axonal transport. Remarkably, these entry and transport mechanisms mirror those used by misfolded proteins implicated in neurodegenerative diseases. Our findings establish the mechanics of reovirus neuronal uptake and spread and provide clues about therapeutic targets to limit neuropathology inflicted by pathogens and misfolded proteins.

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.

A projection specific logic to sampling visual inputs in mouse superior colliculus

Using sensory information to trigger different behaviors relies on circuits that pass through brain regions. The rules by which parallel inputs are routed to downstream targets are poorly understood. The superior colliculus mediates a set of innate behaviors, receiving input from >30 retinal ganglion cell types and projecting to behaviorally important targets including the pulvinar and parabigeminal nucleus. Combining transsynaptic circuit tracing with in vivo and ex vivo electrophysiological recordings, we observed a projection-specific logic where each collicular output pathway sampled a distinct set of retinal inputs. Neurons projecting to the pulvinar or the parabigeminal nucleus showed strongly biased sampling from four cell types each, while six others innervated both pathways. The visual response properties of retinal ganglion cells correlated well with those of their disynaptic targets. These findings open the possibility that projection-specific sampling of retinal inputs forms a basis for the selective triggering of behaviors by the superior colliculus.

The Herpes Simplex Virus 1 Deamidase Enhances Propagation but Is Dispensable for Retrograde Axonal Transport into the Nervous System

Upon replication in mucosal epithelia and transmission to nerve endings, capsids of herpes simplex virus 1 (HSV-1) travel retrogradely within axons to peripheral ganglia, where life-long latent infections are established. A capsid-bound tegument protein, pUL37, is an essential effector of retrograde axonal transport and also houses a deamidase activity that antagonizes innate immune signaling. In this report, we examined whether the deamidase of HSV-1 pUL37 contributes to the neuroinvasive retrograde axonal transport mechanism. We conclude that neuroinvasion is enhanced by the deamidase, but the critical contribution of pUL37 to retrograde axonal transport functions independently of this activity.IMPORTANCE Herpes simplex virus 1 invades the nervous system by entering nerve endings and sustaining long-distance retrograde axonal transport to reach neuronal nuclei in ganglia of the peripheral nervous system. The incoming viral particle carries a deamidase activity on its surface that antagonizes antiviral responses. We examined the contribution of the deamidase to the hallmark neuroinvasive property of this virus.

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.