Microtubule-Dependent Trafficking of Alphaherpesviruses in the Nervous System: The Ins and Outs

Individual herpes simplex virus 1 (HSV-1) particles exit by exocytosis and accumulate at preferential egress sites

The human pathogen herpes simplex virus 1 (HSV-1) produces a lifelong infection in the majority of the world's population. While the generalities of alpha herpesvirus assembly and egress pathways are known, the precise molecular and spatiotemporal details remain unclear. In order to study this aspect of HSV-1 infection, we engineered a recombinant HSV-1 strain expressing a pH-sensitive reporter, gM-pHluorin. Using a variety of fluorescent microscopy modalities, we can detect individual virus particles undergoing intracellular transport and exocytosis at the plasma membrane. We show that particles exit from epithelial cells individually, not bulk release of many particles at once, as has been reported for other viruses. In multiple cell types, HSV-1 particles accumulate over time at the cell periphery and cell-cell contacts. We show that this accumulation effect is the result of individual particles undergoing exocytosis at preferential sites and that these egress sites can contribute to cell-cell spread. We also show that the viral membrane proteins gE, gI, and US9, which have important functions in intracellular transport in neurons, are not required for preferential egress and clustering in non-neuronal cells. Importantly, by comparing HSV-1 to a related alpha herpesvirus, pseudorabies virus, we show that this preferential exocytosis and clustering effect are cell type dependent, not virus dependent. This preferential egress and clustering appear to be the result of the arrangement of the microtubule cytoskeleton, as virus particles co-accumulate at the same cell protrusions as an exogenous plus end-directed kinesin motor.IMPORTANCEAlpha herpesviruses produce lifelong infections in their human and animal hosts. The majority of people in the world are infected with herpes simplex virus 1 (HSV-1), which typically causes recurrent oral or genital lesions. However, HSV-1 can also spread to the central nervous system, causing severe encephalitis, and might also contribute to the development of neurodegenerative diseases. Many of the steps of how these viruses infect and replicate inside host cells are known in depth, but the final step, exiting from the infected cell, is not fully understood. In this study, we engineered a novel variant of HSV-1 that allows us to visualize how individual virus particles exit from infected cells. With this imaging assay, we investigated preferential egress site formation in certain cell types and their contribution to the cell-cell spread of HSV-1.

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.

Brain Epitranscriptomic Analysis Revealed Altered A-to-I RNA Editing in Septic Patients

Recent studies suggest that RNA editing is associated with impaired brain function and neurological and psychiatric disorders. However, the role of A-to-I RNA editing during sepsis-associated encephalopathy (SAE) remains unclear. In this study, we analyzed adenosine-to-inosine (A-to-I) RNA editing in postmortem brain tissues from septic patients and controls. A total of 3024 high-confidence A-to-I RNA editing sites were identified. In sepsis, there were fewer A-to-I RNA editing genes and editing sites than in controls. Among all A-to-I RNA editing sites, 42 genes showed significantly differential RNA editing, with 23 downregulated and 19 upregulated in sepsis compared to controls. Notably, more than 50% of these genes were highly expressed in the brain and potentially related to neurological diseases. Notably, cis-regulatory analysis showed that the level of RNA editing in six differentially edited genes was significantly correlated with the gene expression, including HAUS augmin-like complex subunit 2 (HAUS2), protein phosphatase 3 catalytic subunit beta (PPP3CB), hook microtubule tethering protein 3 (HOOK3), CUB and Sushi multiple domains 1 (CSMD1), methyltransferase-like 7A (METTL7A), and kinesin light chain 2 (KLC2). Furthermore, enrichment analysis showed that fewer gene functions and KEGG pathways were enriched by edited genes in sepsis compared to controls. These results revealed alteration of A-to-I RNA editing in the human brain associated with sepsis, thus providing an important basis for understanding its role in neuropathology in SAE.

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.

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.

Pathogenesis and virulence of herpes simplex virus

Two of the most prevalent human viruses worldwide, herpes simplex virus type 1 and type 2 (HSV-1 and HSV-2, respectively), cause a variety of diseases, including cold sores, genital herpes, herpes stromal keratitis, meningitis and encephalitis. The intrinsic, innate and adaptive immune responses are key to control HSV, and the virus has developed mechanisms to evade them. The immune response can also contribute to pathogenesis, as observed in stromal keratitis and encephalitis. The fact that certain individuals are more prone than others to suffer severe disease upon HSV infection can be partially explained by the existence of genetic polymorphisms in humans. Like all herpesviruses, HSV has two replication cycles: lytic and latent. During lytic replication HSV produces infectious viral particles to infect other cells and organisms, while during latency there is limited gene expression and lack of infectious virus particles. HSV establishes latency in neurons and can cause disease both during primary infection and upon reactivation. The mechanisms leading to latency and reactivation and which are the viral and host factors controlling these processes are not completely understood. Here we review the HSV life cycle, the interaction of HSV with the immune system and three of the best-studied pathologies: Herpes stromal keratitis, herpes simplex encephalitis and genital herpes. We also discuss the potential association between HSV-1 infection and Alzheimer's disease.

Motor Skills: Recruitment of Kinesins, Myosins and Dynein during Assembly and Egress of Alphaherpesviruses

The alphaherpesviruses are pathogens of the mammalian nervous system. Initial infection is commonly at mucosal epithelia, followed by spread to, and establishment of latency in, the peripheral nervous system. During productive infection, viral gene expression, replication of the dsDNA genome, capsid assembly and genome packaging take place in the infected cell nucleus, after which mature nucleocapsids emerge into the cytoplasm. Capsids must then travel to their site of envelopment at cytoplasmic organelles, and enveloped virions need to reach the cell surface for release and spread. Transport at each of these steps requires movement of alphaherpesvirus particles through a crowded and viscous cytoplasm, and for distances ranging from several microns in epithelial cells, to millimeters or even meters during egress from neurons. To solve this challenging problem alphaherpesviruses, and their assembly intermediates, exploit microtubule- and actin-dependent cellular motors. This review focuses upon the mechanisms used by alphaherpesviruses to recruit kinesin, myosin and dynein motors during assembly and egress.

Anterograde transport of α-herpesviruses in neuronal axons

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

HSV-1 Cytoplasmic Envelopment and Egress

Herpes simplex virus type 1 (HSV-1) is a structurally complex enveloped dsDNA virus that has evolved to replicate in human neurons and epithelia. Viral gene expression, DNA replication, capsid assembly, and genome packaging take place in the infected cell nucleus, which mature nucleocapsids exit by envelopment at the inner nuclear membrane then de-envelopment into the cytoplasm. Once in the cytoplasm, capsids travel along microtubules to reach, dock, and envelope at cytoplasmic organelles. This generates mature infectious HSV-1 particles that must then be sorted to the termini of sensory neurons, or to epithelial cell junctions, for spread to uninfected cells. The focus of this review is upon our current understanding of the viral and cellular molecular machinery that enables HSV-1 to travel within infected cells during egress and to manipulate cellular organelles to construct its envelope.

The Role of Herpes Simplex Virus Type 1 Infection in Demyelination of the Central Nervous System

Herpes simplex type 1 (HSV-1) is a neurotropic virus that infects the peripheral and central nervous systems. After primary infection in epithelial cells, HSV-1 spreads retrogradely to the peripheral nervous system (PNS), where it establishes a latent infection in the trigeminal ganglia (TG). The virus can reactivate from the latent state, traveling anterogradely along the axon and replicating in the local surrounding tissue. Occasionally, HSV-1 may spread trans-synaptically from the TG to the brainstem, from where it may disseminate to higher areas of the central nervous system (CNS). It is not completely understood how HSV-1 reaches the CNS, although the most accepted idea is retrograde transport through the trigeminal or olfactory tracts. Once in the CNS, HSV-1 may induce demyelination, either as a direct trigger or as a risk factor, modulating processes such as remyelination, regulation of endogenous retroviruses, or molecular mimicry. In this review, we describe the current knowledge about the involvement of HSV-1 in demyelination, describing the pathways used by this herpesvirus to spread throughout the CNS and discussing the data that suggest its implication in demyelinating processes.

Deletion of the Pseudorabies Virus gE/gI-US9p complex disrupts kinesin KIF1A and KIF5C recruitment during egress, and alters the properties of microtubule-dependent transport in vitro

During infection of neurons by alphaherpesviruses including Pseudorabies virus (PRV) and Herpes simplex virus type 1 (HSV-1) viral nucleocapsids assemble in the cell nucleus, become enveloped in the cell body then traffic into and down axons to nerve termini for spread to adjacent epithelia. The viral membrane protein US9p and the membrane glycoprotein heterodimer gE/gI play critical roles in anterograde spread of both HSV-1 and PRV, and several models exist to explain their function. Biochemical studies suggest that PRV US9p associates with the kinesin-3 motor KIF1A in a gE/gI-stimulated manner, and the gE/gI-US9p complex has been proposed to recruit KIF1A to PRV for microtubule-mediated anterograde trafficking into or along the axon. However, as loss of gE/gI-US9p essentially abolishes delivery of alphaherpesviruses to the axon it is difficult to determine the microtubule-dependent trafficking properties and motor-composition of Δ(gE/gI-US9p) particles. Alternatively, studies in HSV-1 have suggested that gE/gI and US9p are required for the appearance of virions in the axon because they act upstream, to help assemble enveloped virions in the cell body. We prepared Δ(gE/gI-US9p) mutant, and control parental PRV particles from differentiated cultured neuronal or porcine kidney epithelial cells and quantitated the efficiency of virion assembly, the properties of microtubule-dependent transport and the ability of viral particles to recruit kinesin motors. We find that loss of gE/gI-US9p has no significant effect upon PRV particle assembly but leads to greatly diminished plus end-directed traffic, and enhanced minus end-directed and bidirectional movement along microtubules. PRV particles prepared from infected differentiated mouse CAD neurons were found to be associated with either kinesin KIF1A or kinesin KIF5C, but not both. Loss of gE/gI-US9p resulted in failure to recruit KIF1A and KF5C, but did not affect dynein binding. Unexpectedly, while KIF5C was expressed in undifferentiated and differentiated CAD neurons it was only found associated with PRV particles prepared from differentiated cells.