Targeting of pseudorabies virus structural proteins to axons requires association of the viral Us9 protein with lipid rafts

The gE/gI complex is necessary for kinesin-1 recruitment during alphaherpesvirus egress from neurons

Following reactivation of a latent alphaherpesvirus infection, viral particles are assembled in neuronal cell bodies, trafficked anterogradely within axons to nerve termini, and spread to adjacent epithelial cells. The virally encoded membrane proteins US9p and the glycoprotein heterodimer gE/gI of pseudorabies virus (PRV) and herpes simplex virus type 1 (HSV-1) play critical roles in anterograde spread, likely as a tripartite gE/gI-US9p complex. Two kinesin motors, kinesin-1 and kinesin-3, are implicated in the egress of these viruses, but how gE/gI-US9p coordinates their activities is poorly understood. Here, we report that PRV, in addition to associating with the kinesin-3 motor KIF1A, recruits the neuronal kinesin-1 isoforms KIF5A and KIF5C, but not the broadly expressed isoform KIF5B, during egress from differentiated CAD neurons. Similarly, in the axons of dorsal root ganglia (DRG)-derived sensory neurons, PRV colocalized with KIF5C but not KIF5B. In differentiated CAD cells, the association of KIF1A with egressing PRV was dependent upon US9p, whereas the recruitment of KIF5 isoforms required gE/gI. Consistent with these findings, the number of PRV particles trafficking within CAD neurites and the axons of DRG neurons increased when kinesin-1 motor activity was upregulated by hyperacetylating microtubules using trichostatin A (TSA) or tubacin, and this enhanced trafficking depended upon the presence of gE/gI. We propose that, following its recruitment by US9p, KIF1A delivers PRV particles to a location where KIF5 motors are subsequently added by a gE/gI-dependent mechanism. KIF5A/C isoforms then serve to traffic viral particles along axons, resulting in characteristic recrudescent infection.

IMPORTANCE

Alphaherpesviruses include important human and veterinary pathogens that share a unique propensity to establish life-long latent infections in the peripheral nervous system. Upon reactivation, these viruses navigate back to body surfaces and transmit to new hosts. In this study, we demonstrate that the virus gE/gI-US9p membrane complex routes virus particles down this complex neuronal egress pathway by coordinating their association with multiple kinesin microtubule motors.

Recent Advances in the Study of Alphaherpesvirus Latency and Reactivation: Novel Guidance for the Design of Herpesvirus Live Vector Vaccines

Alphaherpesviruses, including herpes simplex virus type 1 (HSV-1), herpes simplex virus type 2 (HSV-2), and varicella-zoster virus (VZV), infect a diverse array of hosts, spanning both humans and animals. Alphaherpesviruses have developed a well-adapted relationship with their hosts through long-term evolution. Some alphaherpesviruses exhibit a typical neurotropic characteristic, which has garnered widespread attention and in-depth research. Virus latency involves the retention of viral genomes without producing infectious viruses. However, under stress, this can be reversed, resulting in lytic infection. Such reactivation events can lead to recurrent infections, manifesting as diseases like herpes labialis, genital herpes, and herpes zoster. Reactivation is a complex process influenced by both viral and host factors, and identifying how latency and reactivation work is vital to developing new antiviral therapies. Recent research highlights a complex interaction among the virus, neurons, and the immune system in regulating alphaherpesvirus latency and reactivation. Neurotropic alphaherpesviruses can breach host barriers to infect neurons, proliferate extensively within their cell bodies, and establish latent infections or spread further. Whether infecting neurons or spreading further, the virus undergoes transmission along axons or dendrites, making this process an indispensable part of the viral life cycle and a critical factor influencing the virus's invasion of the nervous system. Research on the transmission process of neurotropic alphaherpesviruses within neurons can not only deepen our understanding of the virus but can also facilitate the targeted development of corresponding vaccines. This review concentrates on the relationship between the transmission, latency, and activation of alphaherpesviruses within neurons, summarizes recent advancements in the field, and discusses how these findings can inform the design of live virus vaccines for alphaherpesviruses.

Transcriptomic analysis reveals impact of gE/gI/TK deletions on host response to PRV infection

BACKGROUND

Pseudorabies virus (PRV) causes substantial losses in the swine industry worldwide. Attenuated PRV strains with deletions of immunomodulatory genes glycoprotein E (gE), glycoprotein I (gI) and thymidine kinase (TK) are candidate vaccines. However, the effects of gE/gI/TK deletions on PRV-host interactions are not well understood.

METHODS

To characterize the impact of gE/gI/TK deletions on host cells, we analyzed and compared the transcriptomes of PK15 cells infected with wild-type PRV (SD2017), PRV with gE/gI/TK deletions (SD2017gE/gI/TK) using RNA-sequencing.

RESULTS

The attenuated SD2017gE/gI/TK strain showed increased expression of inflammatory cytokines and pathways related to immunity compared to wild-type PRV. Cell cycle regulation and metabolic pathways were also perturbed.

CONCLUSIONS

Deletion of immunomodulatory genes altered PRV interactions with host cells and immune responses. This study provides insights into PRV vaccine design.

Proteomic Comparison of Three Wild-Type Pseudorabies Virus Strains and the Attenuated Bartha Strain Reveals Reduced Incorporation of Several Tegument Proteins in Bartha Virions

Pseudorabies virus (PRV) is a member of the alphaherpesvirus subfamily and the causative agent of Aujeszky's disease in pigs. Driven by the large economic losses associated with PRV infection, several vaccines and vaccine programs have been developed. To this day, the attenuated Bartha strain, generated by serial passaging, represents the golden standard for PRV vaccination. However, a proteomic comparison of the Bartha virion to wild-type (WT) PRV virions is lacking. Here, we present a comprehensive mass spectrometry-based proteome comparison of the attenuated Bartha strain and three commonly used WT PRV strains: Becker, Kaplan, and NIA3. We report the detection of 40 structural and 14 presumed nonstructural proteins through a combination of data-dependent and data-independent acquisition. Interstrain comparisons revealed that packaging of the capsid and most envelope proteins is largely comparable in-between all four strains, except for the envelope protein pUL56, which is less abundant in Bartha virions. However, distinct differences were noted for several tegument proteins. Most strikingly, we noted a severely reduced incorporation of the tegument proteins IE180, VP11/12, pUS3, VP22, pUL41, pUS1, and pUL40 in Bartha virions. Moreover, and likely as a consequence, we also observed that Bartha virions are on average smaller and more icosahedral compared to WT virions. Finally, we detected at least 28 host proteins that were previously described in PRV virions and noticed considerable strain-specific differences with regard to host proteins, arguing that the potential role of packaged host proteins in PRV replication and spread should be further explored. IMPORTANCE The pseudorabies virus (PRV) vaccine strain Bartha-an attenuated strain created by serial passaging-represents an exceptional success story in alphaherpesvirus vaccination. Here, we used mass spectrometry to analyze the Bartha virion composition in comparison to three established WT PRV strains. Many viral tegument proteins that are considered nonessential for viral morphogenesis were drastically less abundant in Bartha virions compared to WT virions. Interestingly, many of the proteins that are less incorporated in Bartha participate in immune evasion strategies of alphaherpesviruses. In addition, we observed a reduced size and more icosahedral morphology of the Bartha virions compared to WT PRV. Given that the Bartha vaccine strain elicits potent immune responses, our findings here suggest that differences in protein packaging may contribute to its immunogenicity. Further exploration of these observations could aid the development of efficacious vaccines against other alphaherpesvirus vaccines such as HSV-1/2 or EHV-1.

Alphaherpesvirus glycoprotein E: A review of its interactions with other proteins of the virus and its application in vaccinology

The viral envelope glycoprotein E (gE) is required for cell-to-cell transmission, anterograde and retrograde neurotransmission, and immune evasion of alphaherpesviruses. gE can also interact with other proteins of the virus and perform various functions in the virus life cycle. In addition, the gE gene is often the target gene for the construction of gene-deleted attenuated marker vaccines. In recent years, new progress has been made in the research and vaccine application of gE with other proteins of the virus. This article reviews the structure of gE, the relationship between gE and other proteins of the virus, and the application of gE in vaccinology, which provides useful information for further research on gE.

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.

Structural Domains of the Herpes Simplex Type 1 gD Protein that Restrict HIV-1 Particle Infectivity

Previously, we showed that the presence of the herpes simplex virus type 1 (HSV-1) gD glycoprotein but not gB potently restricted HIV-1 particle infectivity. This restriction was characterized by incorporation of HSV-1 gD and the exclusion of the HIV-1 gp120/gp41 from budding virus particles. To determine the structural domains involved in gD restriction of HIV-1, a series of deletion mutants and chimeric proteins between gD and the non-restrictive gB were generated. Our results show that deletion of the cytoplasmic tail domain (CTD) of gD or that replacement of the transmembrane domain (TMD) with the TMD from gB slightly reduced restriction activity. However, replacement of the gD CTD with that of gB resulted in lower cell surface expression, significantly less incorporation into HIV-1 particles, and inefficient restriction of the release of infectious HIV-1. Analysis of gB/gD chimeric proteins revealed that removal of the gB CTD or replacement with gD CTD resulted in enhanced surface expression and an increase in restriction activity. Finally, we show that expression of gD without other HSV-1 proteins resulted in gD fractionation into detergent resistant membranes (DRM) and that gD co-localized with the raft marker GM1, which may partially explain its incorporation into budding virus particles. Taken together, our results suggest that expression of gD at the cell surface is likely a major factor but that other intrinsic properties are also involved in the gD-mediated restriction of HIV-1 particle infectivity.IMPORTANCE Previously, we showed that unlike the HSV-1, the presence of the gD glycoprotein in virus producer cells but not gB potently restricted HIV-1 particle infectivity. To better understand the relationship between cell surface expression, virus incorporation and restriction of HIV-1, we analyzed a series of deletion mutants and chimeric proteins in which domains of gD and gB were swapped. Our results indicate that: a) gD/gB chimeras having the cytoplasmic domain (CTD) of gB significantly reduced cell surface expression, release from cells, incorporation into virus, and reduced HIV-1 restriction; b) removal of the gB CTD or replacement with the gD CTD resulted in better surface expression, incorporation into HIV-1, and enhanced restriction; and c) the transmembrane domain of gB can influence transport and ultimately effect incorporation of gB into HIV-1. Overall, these data support a role for gD surface expression as crucial to restriction of infectious HIV-1 release.

Opioid Modulation of Neuronal Iron and Potential Contributions to NeuroHIV

Opioid use has substantially increased over recent years and remains a major driver of new HIV infections worldwide. Clinical studies indicate that opioids may exacerbate the symptoms of HIV-associated neurocognitive disorders (HAND), but the mechanisms underlying opioid-induced cognitive decline remain obscure. We recently reported that the μ-opioid agonist morphine increased neuronal iron levels and levels of ferritin proteins that store iron, suggesting that opioids modulate neuronal iron homeostasis. Additionally, increased iron and ferritin heavy chain protein were necessary for morphine's ability to reduce the density of thin and mushroom dendritic spines in cortical neurons, which are considered critical mediators of learning and memory, respectively. As altered iron homeostasis has been reported in HAND and related neurocognitive disorders like Alzheimer's, Parkinson's, and Huntington's disease, understanding how opioids regulate neuronal iron metabolism may help identify novel drug targets in HAND with potential relevance to these other neurocognitive disorders. Here, we review the known mechanisms of opioid-mediated regulation of neuronal iron and corresponding cellular responses and discuss the implications of these findings for patients with HAND. Furthermore, we discuss a new molecular approach that can be used to understand if opioid modulation of iron affects the expression and processing of amyloid precursor protein and the contributions of this pathway to HAND.

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

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

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.

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.

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

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

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

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

Alpha-herpesviruses represent a group of large, enveloped DNA viruses that are capable to establish a quiescent (also called latent) but reactivatable form of infection in the peripheral nervous system of their hosts. Following reactivation of latent genomes, virus progeny is formed in the soma of neuronal cells and depend on sorting into the axon for anterograde spread of infection to mucosal sites and potentially new host. We studied two alpha-herpesviruses (the veterinary pathogen pseudorabies virus (PRV) and human herpes simplex virus 1 (HSV-1)) and found viral membrane proteins Us7, Us8, and Us9 form a complex, which is able to recruit kinsin-3 motors. Motor recruitment facilitates axonal sorting and subsequent transport to distal egress sites. Complex formation occurs at the trans-Golgi network and mediates efficiency of axonal sorting and motility characteristics of egressing capsids. We also used an artificial kinesin-3 recruitment system, which allows controlled induction of axonal sorting and transport of virus mutants lacking Us7, Us8, and Us9. Overall, these data contribute to our understanding of anterograde alpha-herpesvirus spread and kinesin-mediated sorting of vesicular axonal cargoes.

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.

Proteomics Analysis Identifies IRSp53 and Fascin as Critical for PRV Egress and Direct Cell-Cell Transmission

Pseudorabies virus (PRV) has been widely used as a live trans-synaptic tracer for mapping neuronal circuits. Systematically identifying mature PRV virion proteomes and defining co-purified host proteins are necessary to fully understand the detailed mechanism underlying PRV transmission processes. Here, a PRV virion purification strategy based on sorting with flow cytometry is developed and the mature extracellular and intracellular PRV virion proteomes using LC coupled with MS/MS are characterized. In addition to viral proteins, a large number of host proteins are also identified, including proteins related to actin cytoskeletal dynamics and membrane protrusion. How many of these host proteins are true virion components are unknown and the majority of these may not be. Through functional analysis, it is found that IRSp53 and fascin are critical for the egress process and play a role in direct cell-cell transmission. Moreover, it is shown that CDC42 and Rac1 are also involved in the production of mature extracellular virions. The results suggest that the formation of the filopodia-like cytoskeleton and the rearrangement of the membrane, which are both associated with IRSp53 and fascin, may be important for the transmission of viruses used in neuronal tracing.

Hitchhiking on the neuronal highway: Mechanisms of transsynaptic specificity

Transsynaptic viral tracers are an invaluable neuroanatomical tool to define neuronal circuit connectivity across single or multiple synapses. There are variants that label either inputs or outputs of defined starter populations, most of which are based on the herpes and rabies viruses. However, we still have an incomplete understanding of the factors influencing specificity of neuron-neuron transmission and labeling of inputs vs. outputs. This article will touch on three topics: First, how specific are the directional transmission patterns of these viruses? Second, what are the properties that confer synaptic specificity of viral transmission? Lastly, what can we learn from this specificity, and can we use it to devise better transsynaptic tracers?

Effect of sterol structure on ordered membrane domain (raft) stability in symmetric and asymmetric vesicles

Sterol structure influences liquid ordered domains in membranes, and the dependence of biological functions on sterol structure can help identify processes dependent on ordered domains. In this study we compared the effect of sterol structure on ordered domain formation in symmetric vesicles composed of mixtures of sphingomyelin, 1, 2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and cholesterol, and in asymmetric vesicles in which sphingomyelin was introduced into the outer leaflet of vesicles composed of DOPC and cholesterol. In most cases, sterol behavior was similar in symmetric and asymmetric vesicles, with ordered domains most strongly stabilized by 7-dehydrocholesterol (7DHC) and cholesterol, stabilized to a moderate degree by lanosterol, epicholesterol and desmosterol, and very little if at all by 4-cholesten-3-one. However, in asymmetric vesicles desmosterol stabilized ordered domain almost as well as cholesterol, and to a much greater degree than epicholesterol, so that the ability to support ordered domains decreased in the order 7-DHC > cholesterol > desmosterol > lanosterol > epicholesterol > 4-cholesten-3-one. This contrasts with values for intermediate stabilizing sterols in symmetric vesicles in which the ranking was cholesterol > lanosterol ~ desmosterol ~ epicholesterol or prior studies in which the ranking was cholesterol ~ epicholesterol > lanosterol ~ desmosterol. The reasons for these differences are discussed. Based on these results, we re-evaluated our prior studies in cells and conclude that endocytosis levels and bacterial uptake are even more closely correlated with the ability of sterols to form ordered domains than previously thought, and do not necessarily require that a sterol have a 3β-OH group.

Infection and Transport of Herpes Simplex Virus Type 1 in Neurons: Role of the Cytoskeleton

Herpes simplex virus type 1 (HSV-1) is a neuroinvasive human pathogen that has the ability to infect and replicate within epithelial cells and neurons and establish a life-long latent infection in sensory neurons. HSV-1 depends on the host cellular cytoskeleton for entry, replication, and exit. Therefore, HSV-1 has adapted mechanisms to promote its survival by exploiting the microtubule and actin cytoskeletons to direct its active transport, infection, and spread between neurons and epithelial cells during primary and recurrent infections. This review will focus on the currently known mechanisms utilized by HSV-1 to harness the neuronal cytoskeleton, molecular motors, and the secretory and exocytic pathways for efficient virus entry, axonal transport, replication, assembly, and exit from the distinct functional compartments (cell body and axon) of the highly polarized sensory neurons.

The lipid raft-dwelling protein US9 can be manipulated to target APP compartmentalization, APP processing, and neurodegenerative disease pathogenesis

The trafficking behavior of the lipid raft-dwelling US9 protein from Herpes Simplex Virus strikingly overlaps with that of the amyloid precursor protein (APP). Both US9 and APP processing machinery rely on their ability to shuttle between endosomes and plasma membranes, as well as on their lateral accumulation in lipid rafts. Therefore, repurposing US9 to track/modify these molecular events represents a valid approach to investigate pathological states including Alzheimer's disease and HIV-associated neurocognitive disorders where APP misprocessing to amyloid beta formation has been observed. Accordingly, we investigated the cellular localization of US9-driven cargo in neurons and created a US9-driven functional assay based on the exogenous enzymatic activity of Tobacco Etch Virus Protease. Our results demonstrate that US9 can direct and control cleavage of recombinant proteins exposed on the luminal leaflet of transport vesicles. Furthermore, we confirmed that US9 is associated with lipid-rafts and can target functional enzymes to membrane microdomains where pathologic APP-processing is thought to occur. Overall, our results suggest strongly that US9 can serve as a molecular driver that targets functional cargos to the APP machinery and can be used as a tool to study the contribution of lipid rafts to neurodegenerative disease conditions where amyloidogenesis has been implicated.

Assembly and Egress of an Alphaherpesvirus Clockwork

All viruses produce infectious particles that possess some degree of stability in the extracellular environment yet disassemble upon cell contact and entry. For the alphaherpesviruses, which include many neuroinvasive viruses of mammals, these metastable virions consist of an icosahedral capsid surrounded by a protein matrix (referred to as the tegument) and a lipid envelope studded with glycoproteins. Whereas the capsid of these viruses is a rigid structure encasing the DNA genome, the tegument and envelope are dynamic assemblies that orchestrate a sequential series of events that ends with the delivery of the genome into the nucleus. These particles are adapted to infect two different polarized cell types in their hosts: epithelial cells and neurons of the peripheral nervous system. This review considers how the virion is assembled into a primed state and is targeted to infect these cell types such that the incoming particles can subsequently negotiate the diverse environments they encounter on their way from plasma membrane to nucleus and thereby achieve their remarkably robust neuroinvasive infectious cycle.

Molecular association of herpes simplex virus type 1 glycoprotein E with membrane protein Us9

Herpes simplex virus type 1 (HSV-1) glycoprotein E (gE), glycoprotein I (gI), and Us9 promote efficient anterograde axonal transport of virus from the neuron cytoplasm to the axon terminus. HSV-1 and PRV gE and gI form a heterodimer that is required for anterograde transport, but an association that includes Us9 has not been demonstrated. NS-gE380 is an HSV-1 mutant that has five amino acids inserted after gE residue 380, rendering it defective in anterograde axonal transport. We demonstrated that gE, gI and Us9 form a trimolecular complex in Vero cells infected with NS-gE380 virus in which gE binds to both Us9 and gI. We detected the complex using immunoprecipitation with anti-gE or anti-gI monoclonal antibodies in the presence of ionic detergents. Under these conditions, Us9 did not associate with gE in cells infected with wild-type HSV-1; however, using a nonionic detergent, TritonX-100, an association between Us9 and gE was detected in immunoprecipitates of both wild-type and NS-gE380-infected cells. The results suggest that the interaction between Us9 and gE is weak and disrupted by ionic detergents in wild-type infected cells. We postulate that the tight interaction between Us9 and gE leads to the anterograde spread defect in the NS-gE380 virus.

Dual Role of Herpes Simplex Virus 1 pUS9 in Virus Anterograde Axonal Transport and Final Assembly in Growth Cones in Distal Axons

The herpes simplex virus type 1 (HSV-1) envelope protein pUS9 plays an important role in virus anterograde axonal transport and spread from neuronal axons. In this study, we used both confocal microscopy and transmission electron microscopy (TEM) to examine the role of pUS9 in the anterograde transport and assembly of HSV-1 in the distal axon of human and rat dorsal root ganglion (DRG) neurons using US9 deletion (US9(-)), repair (US9R), and wild-type (strain F, 17, and KOS) viruses. Using confocal microscopy and single and trichamber culture systems, we observed a reduction but not complete block in the anterograde axonal transport of capsids to distal axons as well as a marked (∼90%) reduction in virus spread from axons to Vero cells with the US9 deletion viruses. Axonal transport of glycoproteins (gC, gD, and gE) was unaffected. Using TEM, there was a marked reduction or absence of enveloped capsids, in varicosities and growth cones, in KOS strain and US9 deletion viruses, respectively. Capsids (40 to 75%) in varicosities and growth cones infected with strain 17, F, and US9 repair viruses were fully enveloped compared to less than 5% of capsids found in distal axons infected with the KOS strain virus (which also lacks pUS9) and still lower (<2%) with the US9 deletion viruses. Hence, there was a secondary defect in virus assembly in distal axons in the absence of pUS9 despite the presence of key envelope proteins. Overall, our study supports a dual role for pUS9, first in anterograde axonal transport and second in virus assembly in growth cones in distal axons.

IMPORTANCE

HSV-1 has evolved mechanisms for its efficient transport along sensory axons and subsequent spread from axons to epithelial cells after reactivation. In this study, we show that deletion of the envelope protein pUS9 leads to defects in virus transport along axons (partial defect) and in virus assembly and egress from growth cones (marked defect). Virus assembly and exit in the neuronal cell body are not impaired in the absence of pUS9. Thus, our findings indicate that pUS9 contributes to the overall HSV-1 anterograde axonal transport, including a major role in virus assembly at the axon terminus, which is not essential in the neuronal cell body. Overall, our data suggest that the process of virus assembly at the growth cones differs from that in the neuronal cell body and that HSV-1 has evolved different mechanisms for virus assembly and exit from different cellular compartments.

The Basic Domain of Herpes Simplex Virus 1 pUS9 Recruits Kinesin-1 To Facilitate Egress from Neurons

The alphaherpesviral envelope protein pUS9 has been shown to play a role in the anterograde axonal transport of herpes simplex virus 1 (HSV-1), yet the molecular mechanism is unknown. To address this, we used an in vitro pulldown assay to define a series of five arginine residues within the conserved pUS9 basic domain that were essential for binding the molecular motor kinesin-1. The mutation of these pUS9 arginine residues to asparagine blocked the binding of both recombinant and native kinesin-1. We next generated HSV-1 with the same pUS9 arginine residues mutated to asparagine (HSV-1pUS9KBDM) and then restored them being to arginine (HSV-1pUS9KBDR). The two mutated viruses were analyzed initially in a zosteriform model of recurrent cutaneous infection. The primary skin lesion scores were identical in severity and kinetics, and there were no differences in viral load at dorsal root ganglionic (DRG) neurons at day 4 postinfection (p.i.) for both viruses. In contrast, HSV-1pUS9KBDM showed a partial reduction in secondary skin lesions at day 8 p.i. compared to the level for HSV-1pUS9KBDR. The use of rat DRG neuronal cultures in a microfluidic chamber system showed both a reduction in anterograde axonal transport and spread from axons to nonneuronal cells for HSV-1pUS9KBDM. Therefore, the basic domain of pUS9 contributes to anterograde axonal transport and spread of HSV-1 from neurons to the skin through recruitment of kinesin-1.

IMPORTANCE

Herpes simplex virus 1 and 2 cause genital herpes, blindness, encephalitis, and occasionally neonatal deaths. There is also increasing evidence that sexually transmitted genital herpes increases HIV acquisition, and the reactivation of HSV increases HIV replication and transmission. New antiviral strategies are required to control resistant viruses and to block HSV spread, thereby reducing HIV acquisition and transmission. These aims will be facilitated through understanding how HSV is transported down nerves and into skin. In this study, we have defined how a key viral protein plays a role in both axonal transport and spread of the virus from nerve cells to the skin.

Us9-Independent Axonal Sorting and Transport of the Pseudorabies Virus Glycoprotein gM

Axonal sorting and transport of fully assembled pseudorabies virus (PRV) virions is dependent on the viral protein Us9. Here we identify a Us9-independent mechanism for axonal localization of viral glycoprotein M (gM). We detected gM-mCherry assemblies transporting in the anterograde direction in axons. Furthermore, unlabeled gM, but not glycoprotein B, was detected by Western blotting in isolated axons during Us9-null PRV infection. These results suggest that gM differs from other viral proteins regarding axonal transport properties.

Molecular features contributing to virus-independent intracellular localization and dynamic behavior of the herpesvirus transport protein US9

Reaching the right destination is of vital importance for molecules, proteins, organelles, and cargoes. Thus, intracellular traffic is continuously controlled and regulated by several proteins taking part in the process. Viruses exploit this machinery, and viral proteins regulating intracellular transport have been identified as they represent valuable tools to understand and possibly direct molecules targeting and delivery. Deciphering the molecular features of viral proteins contributing to (or determining) this dynamic phenotype can eventually lead to a virus-independent approach to control cellular transport and delivery. From this virus-independent perspective we looked at US9, a virion component of Herpes Simplex Virus involved in anterograde transport of the virus inside neurons of the infected host. As the natural cargo of US9-related vesicles is the virus (or its parts), defining its autonomous, virus-independent role in vesicles transport represents a prerequisite to make US9 a valuable molecular tool to study and possibly direct cellular transport. To assess the extent of this autonomous role in vesicles transport, we analyzed US9 behavior in the absence of viral infection. Based on our studies, Us9 behavior appears similar in different cell types; however, as expected, the data we obtained in neurons best represent the virus-independent properties of US9. In these primary cells, transfected US9 mostly recapitulates the behavior of US9 expressed from the viral genome. Additionally, ablation of two major phosphorylation sites (i.e. Y32Y33 and S34ES36) have no effect on protein incorporation on vesicles and on its localization on both proximal and distal regions of the cells. These results support the idea that, while US9 post-translational modification may be important to regulate cargo loading and, consequently, virion export and delivery, no additional viral functions are required for US9 role in intracellular transport.

Human cytomegalovirus pUL37x1-induced calcium flux activates PKCα, inducing altered cell shape and accumulation of cytoplasmic vesicles

The human cytomegalovirus immediate-early protein pUL37x1 induces the release of Ca(2+) stores from the endoplasmic reticulum into the cytosol. This release causes reorganization of the cellular actin cytoskeleton with concomitant cell rounding. Here we demonstrate that pUL37x1 activates Ca(2+)-dependent protein kinase Cα (PKCα). Both PKCα and Rho-associated protein kinases are required for actin reorganization and cell rounding; however, only PKCα is required for the efficient production of virus progeny, arguing that HCMV depends on the kinase for a second function. PKCα activation is also needed for the production of large (1-5 μm) cytoplasmic vesicles late after infection. The production of these vesicles is blocked by inhibition of fatty acid or phosphatidylinositol-3-phosphate biosynthesis, and the failure to produce vesicles is correlated with substantially reduced production of enveloped virus capsids. These results connect earlier work identifying a requirement for lipid synthesis with specific morphological changes, and support the argument that the PKCα-induced large vesicles are either required for the efficient production of mature virus particles or serve as a marker for the process.

Glycoproteins gE and gI are required for efficient KIF1A-dependent anterograde axonal transport of alphaherpesvirus particles in neurons

Alphaherpesviruses, including pseudorabies virus (PRV), spread directionally within the nervous systems of their mammalian hosts. Three viral membrane proteins are required for efficient anterograde-directed spread of infection in neurons, including Us9 and a heterodimer composed of the glycoproteins gE and gI. We previously demonstrated that the kinesin-3 motor KIF1A mediates anterograde-directed transport of viral particles in axons of cultured peripheral nervous system (PNS) neurons. The PRV Us9 protein copurifies with KIF1A, recruiting the motor to transport vesicles, but at least one unidentified additional viral protein is necessary for this interaction. Here we show that gE/gI are required for efficient anterograde transport of viral particles in axons by mediating the interaction between Us9 and KIF1A. In the absence of gE/gI, viral particles containing green fluorescent protein (GFP)-tagged Us9 are assembled in the cell body but are not sorted efficiently into axons. Importantly, we found that gE/gI are necessary for efficient copurification of KIF1A with Us9, especially at early times after infection. We also constructed a PRV recombinant that expresses a functional gE-GFP fusion protein and used affinity purification coupled with mass spectrometry to identify gE-interacting proteins. Several viral and host proteins were found to associate with gE-GFP. Importantly, both gI and Us9, but not KIF1A, copurified with gE-GFP. We propose that gE/gI are required for efficient KIF1A-mediated anterograde transport of viral particles because they indirectly facilitate or stabilize the interaction between Us9 and KIF1A.

The Us2 gene product of herpes simplex virus 2 is a membrane-associated ubiquitin-interacting protein

The Us2 gene encodes a tegument protein that is conserved in most members of the Alphaherpesvirinae. Previous studies on the pseudorabies virus (PRV) Us2 ortholog indicated that it is prenylated, associates with membranes, and spatially regulates the enzymatic activity of the MAP (mitogen-activated protein) kinase ERK (extracellular signal-related kinase) through direct binding and sequestration of ERK at the cytoplasmic face of the plasma membrane. Here we present an analysis of the herpes simplex virus 2 (HSV-2) Us2 ortholog and demonstrate that, like PRV Us2, HSV-2 Us2 is a virion component and that, unlike PRV Us2, it does not interact with ERK in yeast two-hybrid assays. HSV-2 Us2 lacks prenylation signals and other canonical membrane-targeting motifs yet is tightly associated with detergent-insoluble membranes and localizes predominantly to recycling endosomes. Experiments to identify cellular proteins that facilitate HSV-2 Us2 membrane association were inconclusive; however, these studies led to the identification of HSV-2 Us2 as a ubiquitin-interacting protein, providing new insight into the functions of HSV-2 Us2.

Kinesin-3 mediates axonal sorting and directional transport of alphaherpesvirus particles in neurons

During infection of the nervous system, alphaherpesviruses-including pseudorabies virus (PRV)-use retrograde axonal transport to travel toward the neuronal cell body and anterograde transport to traffic back to the cell periphery upon reactivation from latency. The PRV protein Us9 plays an essential but unknown role in anterograde viral spread. To determine Us9 function, we identified viral and host proteins that interact with Us9 and explored the role of KIF1A, a microtubule-dependent kinesin-3 motor involved in axonal sorting and transport. Viral particles are cotransported with KIF1A in axons of primary rat superior cervical ganglion neurons, and overexpression or disruption of KIF1A function, respectively, increases and reduces anterograde capsid transport. Us9 and KIF1A interact early during infection with the aid of additional viral protein(s) but exhibit diminished binding at later stages, when capsids typically stall in axons. Thus, alphaherpesviruses repurpose the axonal transport and sorting pathway to spread within their hosts.

Role of Us9 phosphorylation in axonal sorting and anterograde transport of pseudorabies virus

Alphaherpes viruses, such as pseudorabies virus (PRV), undergo anterograde transport in neuronal axons to facilitate anterograde spread within hosts. Axonal sorting and anterograde transport of virions is dependent on the viral membrane protein Us9, which interacts with the host motor protein Kif1A to direct transport. Us9-Kif1A interactions are necessary but not sufficient for these processes, indicating that additional cofactors or post-translational modifications are needed. In this study, we characterized two conserved serine phosphorylation sites (S51 and S53) in the PRV Us9 protein that are necessary for anterograde spread in vivo. We assessed the subcellular localization of phospho-Us9 subspecies during infection of neurons and found that the phospho-form is detectable on the majority, but not all, of axonal vesicles containing Us9 protein. In biochemical assays, phospho-Us9 was enriched in lipid raft membrane microdomains, though Us9 phosphorylation did not require prior lipid raft association. During infections of chambered neuronal cultures, we observed only a modest reduction in anterograde spread capacity for diserine mutant Us9, and no defect for monoserine mutants. Conversely, mutation of the kinase recognition sequence residues adjacent to the phosphorylation sites completely abrogated anterograde spread. In live-cell imaging analyses, anterograde transport of virions was reduced during infection with a recombinant PRV strain expressing GFP-tagged diserine mutant Us9. Phosphorylation was not required for Us9-Kif1A interaction, suggesting that Us9-Kif1A binding is a distinct step from the activation and/or stabilization of the transport complex. Taken together, our findings indicate that, while not essential, Us9 phosphorylation enhances Us9-Kif1A-based transport of virions in axons to modulate the overall efficiency of long-distance anterograde spread of infection.

Directional spread of alphaherpesviruses in the nervous system

Alphaherpesviruses are pathogens that invade the nervous systems of their mammalian hosts. Directional spread of infection in the nervous system is a key component of the viral lifecycle and is critical for the onset of alphaherpesvirus-related diseases. Many alphaherpesvirus infections originate at peripheral sites, such as epithelial tissues, and then enter neurons of the peripheral nervous system (PNS), where lifelong latency is established. Following reactivation from latency and assembly of new viral particles, the infection typically spreads back out towards the periphery. These spread events result in the characteristic lesions (cold sores) commonly associated with herpes simplex virus (HSV) and herpes zoster (shingles) associated with varicella zoster virus (VZV). Occasionally, the infection spreads transsynaptically from the PNS into higher order neurons of the central nervous system (CNS). Spread of infection into the CNS, while rarer in natural hosts, often results in severe consequences, including death. In this review, we discuss the viral and cellular mechanisms that govern directional spread of infection in the nervous system. We focus on the molecular events that mediate long distance directional transport of viral particles in neurons during entry and egress.

Sphingolipids and membrane domains: recent advances

There is growing evidence that cell membranes can contain domains with different lipid and protein compositions and with different physical properties. Furthermore, it is increasingly appreciated that sphingolipids play a crucial role in the formation and properties of ordered lipid domains (rafts) in cell membranes. This review describes recent advances in our understanding of ordered membrane domains in both cells and model membranes. In addition, how the structure of sphingolipids influences their ability to participate in the formation of ordered domains, as well as how sphingolipid structure alters ordered domain properties, is described. The diversity of sphingolipid structure is likely to play an important role in modulating the biologically relevant properties of "rafts" in cell membranes.

Alphaherpesvirus infection disrupts mitochondrial transport in neurons

Mitochondria are dynamic organelles that are essential for cellular metabolism but can be functionally disrupted during pathogen infection. In neurons, mitochondria are transported on microtubules via the molecular motors kinesin-1 and dynein and recruited to energy-requiring regions such as synapses. Previous studies showed that proteins from pseudorabies virus (PRV), an alphaherpesvirus, localize to mitochondria and affect mitochondrial function. We show that PRV and herpes simplex virus type 1 (HSV-1) infection of rodent superior cervical ganglion (SCG) neurons disrupts mitochondrial motility and morphology. During PRV infection, glycoprotein B (gB)-dependent fusion events result in electrical coupling of neurons and increased action potential firing rates. Consequently, intracellular [Ca(2+)] increases and alters mitochondrial dynamics through a mechanism involving the Ca(2+)-sensitive cellular protein Miro and reduced recruitment of kinesin-1 to mitochondria. This disruption in mitochondrial dynamics is required for efficient growth and spread of PRV, indicating that altered mitochondrial transport enhances alphaherpesvirus pathogenesis and infection.

Visualization of an alphaherpesvirus membrane protein that is essential for anterograde axonal spread of infection in neurons

Pseudorabies virus (PRV), an alphaherpesvirus with a broad host range, replicates and spreads in chains of synaptically connected neurons. The PRV protein Us9 is a small membrane protein that is highly conserved among alphaherpesviruses and is essential for anterograde axonal spread in neurons. Specifically, the Us9 protein is required for the sorting of newly assembled PRV particles into axons. However, the molecular details underlying the function of Us9 are poorly understood. Here we constructed PRV strains that express functional green fluorescent protein (GFP)-Us9 fusion proteins in order to visualize axonal transport of viral particles in infected rat superior cervical ganglion neurons. We show that GFP-Us9-labeled structures are transported exclusively in the anterograde direction within axons. Additionally, the vast majority of anterograde-directed capsids (labeled with VP26-monomeric red fluorescent protein) and a viral membrane protein (labeled with glycoprotein M fused to mCherry) are cotransported with GFP-Us9 in the anterograde direction. In contrast, during infection with PRV strains that express nonfunctional mutant GFP-Us9 proteins, cotransport of mutant GFP-Us9 with capsids in axons is abolished. These findings show that axonal sorting of progeny viral particles is dependent upon the association of viral structures with membranes that contain functional Us9 proteins. This association is required for anterograde spread of infection in neurons.

IMPORTANCE

Alphaherpesviruses, such as pseudorabies virus (PRV), are parasites of the mammalian nervous system. These viruses spread over long distances in chains of synaptically connected neurons. PRV encodes several proteins that mediate directed virion transport and spread of infection. Us9 is a highly conserved viral membrane protein that is essential for anterograde neuronal spread of infection. In the absence of Us9, newly replicated viral particles are assembled in the cell body but are not sorted into or transported within axons. Here, we constructed and characterized novel PRV strains that express functional green fluorescent protein (GFP)-Us9 fusion proteins in order to visualize its localization in living neurons during infection. This enabled us to better understand the function of Us9 in facilitating the spread of infection. We show that all viral particles moving in the anterograde direction are labeled with GFP-Us9, suggesting that the presence of Us9 determines the capacity for directed transport within axons.

Cryo electron tomography of herpes simplex virus during axonal transport and secondary envelopment in primary neurons

During herpes simplex virus 1 (HSV1) egress in neurons, viral particles travel from the neuronal cell body along the axon towards the synapse. Whether HSV1 particles are transported as enveloped virions as proposed by the ‘married’ model or as non-enveloped capsids suggested by the ‘separate’ model is controversial. Specific viral proteins may form a recruitment platform for microtubule motors that catalyze such transport. However, their subviral location has remained elusive. Here we established a system to analyze herpesvirus egress by cryo electron tomography. At 16 h post infection, we observed intra-axonal transport of progeny HSV1 viral particles in dissociated hippocampal neurons by live-cell fluorescence microscopy. Cryo electron tomography of frozen-hydrated neurons revealed that most egressing capsids were transported independently of the viral envelope. Unexpectedly, we found not only DNA-containing capsids (cytosolic C-capsids), but also capsids lacking DNA (cytosolic A-/B-capsids) in mid-axon regions. Subvolume averaging revealed lower amounts of tegument on cytosolic A-/B-capsids than on C-capsids. Nevertheless, all capsid types underwent active axonal transport. Therefore, even few tegument proteins on the capsid vertices seemed to suffice for transport. Secondary envelopment of capsids was observed at axon terminals. On their luminal face, the enveloping vesicles were studded with typical glycoprotein-like spikes. Furthermore, we noted an accretion of tegument density at the concave cytosolic face of the vesicle membrane in close proximity to the capsids. Three-dimensional analysis revealed that these assembly sites lacked cytoskeletal elements, but that filamentous actin surrounded them and formed an assembly compartment. Our data support the ‘separate model’ for HSV1 egress, i.e. progeny herpes viruses being transported along axons as subassemblies and not as complete virions within transport vesicles.

Herpes simplex virus 1 (HSV1) establishes lifelong latent infections in the peripheral nervous system. After reactivation, progeny viral particles travel within sensory neurons towards sites of initial infection. There are conflicting reports what type of viral structures are transported: some studies observed non-enveloped capsids traveling while others reported transport of fully enveloped viruses within vesicles. Here, we used cryo electron tomography to analyze the three-dimensional architecture of HSV1 in axons of hippocampal neurons. In mid-axonal regions we found predominantly non-enveloped capsids. Interestingly, we observed both genome-containing and empty capsids that differed significantly in the amount of bound proteins. Viral protein recruitment thus varied between the different cytosolic capsid types, but effective transport occurred despite these differences. Furthermore, we observed three-dimensional snapshots of secondary capsid envelopment in axon terminals. Altogether, this study provides valuable structural detail on axonal HSV1 particles supporting the notion that viral subassemblies are conveyed along the axons to be assembled only after axonal transport.

Function of membrane rafts in viral lifecycles and host cellular response

Membrane rafts are small (10–200 nm) sterol- and sphingolipid-enriched domains that compartmentalize cellular processes. Membrane rafts play an important role in viral infection cycles and viral virulence. Viruses are divided into four main classes, enveloped DNA virus, enveloped RNA virus, nonenveloped DNA virus, and nonenveloped RNA virus. General virus infection cycle is also classified into two sections, the early stage (entry process) and the late stage (assembly, budding, and release processes of virus particles). In the viral cycle, membrane rafts act as a scaffold of many cellular signal transductions, which are associated with symptoms caused by viral infections. In this paper, we describe the functions of membrane rafts in viral lifecycles and host cellular response according to each virus classification, each stage of the virus lifecycle, and each virus-induced signal transduction.

Alphaherpesviruses and the cytoskeleton in neuronal infections

Following infection of exposed peripheral tissues, neurotropic alphaherpesviruses invade nerve endings and deposit their DNA genomes into the nuclei of neurons resident in ganglia of the peripheral nervous system. The end result of these events is the establishment of a life-long latent infection. Neuroinvasion typically requires efficient viral transmission through a polarized epithelium followed by long-distance transport through the viscous axoplasm. These events are mediated by the recruitment of the cellular microtubule motor proteins to the intracellular viral particle and by alterations to the cytoskeletal architecture. The focus of this review is the interplay between neurotropic herpesviruses and the cytoskeleton.

The bovine herpesvirus type 1 envelope protein Us9 acidic domain is crucial for anterograde axonal transport

In this study, we examined the functional role of bovine herpesvirus type 1 (BHV-1) Us9 acidic domain residues 83-90 in the anterograde axonal transport of the virus in calves (natural host), rabbits, and in cultured neurons. A mutant virus strain lacking Us9 residues 83-90 (BHV-1 Us9 Δ83-90) and the rescued virus (BHV-1 Us9 R83-90) replicated efficiently in the nasal and ocular epithelium during primary infection and established latency in the trigeminal ganglia (TG). However, upon reactivation from latency, only the BHV-1 Us9 R83-90 virus was detected in nasal and ocular swabs of animals. In compartmentalized, rabbit primary dorsal root ganglia (DRG) neuron cultures, the Us9-deleted BHV-1, BHV-1 Us9 Δ83-90 and BHV-1 Us9 R83-90 viruses were transported efficiently in the retrograde direction. However, only the BHV-1 Us9 R83-90 virus was transported in an anterograde direction. These studies suggested that the Us9 acidic domain residues located between 83 and 90 were required for axonal anterograde transport.

Herpes simplex virus type 2 glycoprotein E is required for efficient virus spread from epithelial cells to neurons and for targeting viral proteins from the neuron cell body into axons

The HSV-2 lifecycle involves virus spread in a circuit from the inoculation site to dorsal root ganglia and return. We evaluated the role of gE-2 in the virus lifecycle by deleting amino acids 124-495 (gE2-del virus). In the mouse retina infection model, gE2-del virus does not spread to nuclei in the brain, indicating a defect in anterograde (pre-synaptic to post-synaptic neurons) and retrograde (post-synaptic to pre-synaptic neurons) spread. Infection of neuronal cells in vitro demonstrates that gE-2 is required for targeting viral proteins from neuron cell bodies into axons, and for efficient virus spread from epithelial cells to axons. The mouse flank model confirms that gE2-del virus is defective in spread from epithelial cells to neurons. Therefore, we defined two steps in the virus lifecycle that involve gE-2, including efficient spread from epithelial cells to axons and targeting viral components from neuron cell bodies into axons.

Relationships between plasma membrane microdomains and HIV-1 assembly

Advances in cell biology and biophysics revealed that cellular membranes consist of multiple microdomains with specific sets of components such as lipid rafts and TEMs (tetraspanin-enriched microdomains). An increasing number of enveloped viruses have been shown to utilize these microdomains during their assembly. Among them, association of HIV-1 (HIV type 1) and other retroviruses with lipid rafts and TEMs within the PM (plasma membrane) is well documented. In this review, I describe our current knowledge on interrelationships between PM microdomain organization and the HIV-1 particle assembly process. Microdomain association during virus particle assembly may also modulate subsequent virus spread. Potential roles played by microdomains will be discussed with regard to two post-assembly events, i.e., inhibition of virus release by a raft-associated protein BST-2/tetherin and cell-to-cell HIV-1 transmission at virological synapses.

Ultrastructural analysis of virion formation and anterograde intraaxonal transport of the alphaherpesvirus pseudorabies virus in primary neurons

A hallmark of alphaherpesviruses is their capacity to be neuroinvasive and establish latent infections in neurons. After primary replication in epithelial cells at the periphery, entry into nerve endings occurs, followed by retrograde transport of nucleocapsids to the nucleus where viral transcription, genome replication, and nucleocapsid formation take place. Translocation of nucleocapsids to the cytoplasm is followed by axonal transport to infect synaptically linked neurons. Two modes of intraaxonal anterograde herpesvirus transport have been proposed: transport of complete, enveloped virions within vesicles ("married model"), and separate transport of capsids and envelopes ("subassembly model"). To assess this in detail for the alphaherpesvirus pseudorabies virus (PrV), we used high-resolution transmission electron microscopy of primary neuronal cultures from embryonic rat superior cervical ganglia after infection with wild-type and gB-deficient PrV. Our data show that intranuclear capsid maturation, nuclear egress and cytoplasmic secondary envelopment occur as in cultured nonpolarized cells (H. Granzow, F. Weiland, A. Jöns, B. G. Klupp, A. Karger, and T. C. Mettenleiter, J. Virol. 71:2072-2082, 1997). PrV virions were present in axons as enveloped particles within vesicles associated with microtubules and apparently leave the neuron by exocytosis primarily at the growth cone. Only a few nonenveloped nucleocapsids were found in the axon. The same picture was observed after infection by phenotypically complemented gB-deficient PrV, which is able to complete only a single round of replication. Our data thus support intraaxonal anterograde transport of enveloped PrV virions within vesicles following the "married model."

Alphaherpesvirus use and misuse of cellular actin and cholesterol

Two major structural elements of a cell are the cytoskeleton and the lipid membranes. Actin and cholesterol are key components of the cytoskeleton and membranes, respectively, and are involved in a plethora of different cellular processes. This review summarizes and discusses the interaction of alphaherpesviruses with actin and cholesterol during different stages of the replication cycle: virus entry, replication and assembly in the nucleus, and virus egress. Elucidating these interactions not only yields novel insights into the biology of these important pathogens, but may also shed new light on cell biological aspects of actin and cholesterol, and lead to novel avenues in the design of antiviral strategies.

Directional transneuronal spread of α-herpesvirus infection

Most α-herpesviruses are pantropic, neuroinvasive pathogens that establish a reactivateable, latent infection in the PNS of their natural hosts. Various manifestations of herpes disease rely on extent and direction of the spread of infection between the surface epithelia and the nervous system components that innervate that surface. One aspect of such controlled spread of infection is the capacity for synaptically defined, transneuronal spread, a property that makes α-herpesviruses useful tools for determining the connectivity of neural circuits. The current understanding of intra-axonal transport and transneuronal spread of α-herpesviruses is reviewed, focusing on work with herpes simplex virus and pseudorabies virus, the available in vitro technology used to study viral transport and spread is evaluated and how certain viral mutants can be used to examine neural circuit architecture is described in this article.

Pseudorabies virus infection alters neuronal activity and connectivity in vitro

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

Anterograde spread of herpes simplex virus type 1 requires glycoprotein E and glycoprotein I but not Us9

Anterograde neuronal spread (i.e., spread from the neuron cell body toward the axon terminus) is a critical component of the alphaherpesvirus life cycle. Three viral proteins, gE, gI, and Us9, have been implicated in alphaherpesvirus anterograde spread in several animal models and neuron culture systems. We sought to better define the roles of gE, gI, and Us9 in herpes simplex virus type 1 (HSV-1) anterograde spread using a compartmentalized primary neuron culture system. We found that no anterograde spread occurred in the absence of gE or gI, indicating that these proteins are essential for HSV-1 anterograde spread. However, we did detect anterograde spread in the absence of Us9 using two independent Us9-deleted viruses. We confirmed the Us9 finding in different murine models of neuronal spread. We examined viral transport into the optic nerve and spread to the brain after retinal infection; the production of zosteriform disease after flank inoculation; and viral spread to the spinal cord after flank inoculation. In all models, anterograde spread occurred in the absence of Us9, although in some cases at reduced levels. This finding contrasts with gE- and gI-deleted viruses, which displayed no anterograde spread in any animal model. Thus, gE and gI are essential for HSV-1 anterograde spread, while Us9 is dispensable.

Virion-incorporated glycoprotein B mediates transneuronal spread of pseudorabies virus

Transneuronal spread of pseudorabies virus (PRV) is a multistep process that requires several virally encoded proteins. Previous studies have shown that PRV glycoprotein B (gB), a component of the viral fusion machinery, is required for the transmission of infection to postsynaptic, second-order neurons. We sought to identify the gB-mediated step in viral transmission. We determined that gB is not required for the sorting of virions into axons of infected neurons, anterograde transport, or the release of virions from the axon. trans or cis expression of gB on the cell surface was not sufficient for transneuronal spread of the virus; instead, efficient incorporation of gB into virions was required. Additionally, neuron-to-cell spread of PRV most likely does not proceed through syncytial connections. We conclude that, upon gB-independent release of virions at the site of neuron-cell contacts, the virion-incorporated gB/gH/gL fusion complex mediates entry into the axonally contacted cell by fusion of the closely apposed membranes.

Comparison of the pseudorabies virus Us9 protein with homologs from other veterinary and human alphaherpesviruses

Pseudorabies virus (PRV) Us9 is a small, tail-anchored (TA) membrane protein that is essential for axonal sorting of viral structural proteins and is highly conserved among other members of the alphaherpesvirus subfamily. We cloned the Us9 homologs from two human pathogens, varicella-zoster virus (VZV) and herpes simplex virus type 1 (HSV-1), as well as two veterinary pathogens, equine herpesvirus type 1 (EHV-1) and bovine herpesvirus type 1 (BHV-1), and fused them to enhanced green fluorescent protein to examine their subcellular localization and membrane topology. Akin to PRV Us9, all of the Us9 homologs localized to the trans-Golgi network and had a type II membrane topology (typical of TA proteins). Furthermore, we examined whether any of the Us9 homologs could compensate for the loss of PRV Us9 in anterograde, neuron-to-cell spread of infection in a compartmented chamber system. EHV-1 and BHV-1 Us9 were able to fully compensate for the loss of PRV Us9, whereas VZV and HSV-1 Us9 proteins were unable to functionally replace PRV Us9 when they were expressed in a PRV background.

Lipids and membrane microdomains in HIV-1 replication

Several critical steps in the replication cycle of human immunodeficiency virus type 1 (HIV-1) - entry, assembly and budding - are complex processes that take place at the plasma membrane of the host cell. A growing body of data indicates that these early and late steps in HIV-1 replication take place in specialized plasma membrane microdomains, and that many of the viral and cellular components required for entry, assembly, and budding are concentrated in these microdomains. In particular, a number of studies have shown that cholesterol- and sphingolipid-enriched microdomains known as lipid rafts play important roles in multiple steps in the virus replication cycle. In this review, we provide an overview of what is currently known about the involvement of lipids and membrane microdomains in HIV-1 replication.