Cellular mechanisms of alpha herpesvirus egress: live cell fluorescence microscopy of pseudorabies virus exocytosis

Viral interactions with host cell Rab GTPases

Viruses are obligate intracellular parasites that utilize cellular machinery for many aspects of their replication cycles. Enveloped viruses generally rely upon host vesicular trafficking machinery to direct their structural proteins and genomes to sites of virus replication, assembly, and budding. Rab GTPases have been implicated in the replication of many important viral pathogens infecting humans. This review provides a summary of virus-Rab protein interactions, with a particular focus on the role of Rab-related trafficking pathways on late events in the lifecycle of herpesviruses and of HIV-1.

Microtubule Regulation and Function during Virus Infection

Microtubules (MTs) form a rapidly adaptable network of filaments that radiate throughout the cell. These dynamic arrays facilitate a wide range of cellular processes, including the capture, transport, and spatial organization of cargos and organelles, as well as changes in cell shape, polarity, and motility. Nucleating from MT-organizing centers, including but by no means limited to the centrosome, MTs undergo rapid transitions through phases of growth, pause, and catastrophe, continuously exploring and adapting to the intracellular environment. Subsets of MTs can become stabilized in response to environmental cues, acquiring distinguishing posttranslational modifications and performing discrete functions as specialized tracks for cargo trafficking. The dynamic behavior and organization of the MT array is regulated by MT-associated proteins (MAPs), which include a subset of highly specialized plus-end-tracking proteins (+TIPs) that respond to signaling cues to alter MT behavior. As pathogenic cargos, viruses require MTs to transport to and from their intracellular sites of replication. While interactions with and functions for MT motor proteins are well characterized and extensively reviewed for many viruses, this review focuses on MT filaments themselves. Changes in the spatial organization and dynamics of the MT array, mediated by virus- or host-induced changes to MT regulatory proteins, not only play a central role in the intracellular transport of virus particles but also regulate a wider range of processes critical to the outcome of infection.

Mildly Acidic pH Triggers an Irreversible Conformational Change in the Fusion Domain of Herpes Simplex Virus 1 Glycoprotein B and Inactivation of Viral Entry

Herpes simplex virus (HSV) entry into a subset of cells requires endocytosis and endosomal low pH. Preexposure of isolated virions to mildly acidic pH of 5 to 6 partially inactivates HSV infectivity in an irreversible manner. Acid inactivation is a hallmark of viruses that enter via low-pH pathways; this occurs by pretriggering conformational changes essential for fusion. The target and mechanism(s) of low-pH inactivation of HSV are unclear. Here, low-pH-treated HSV-1 was defective in fusion activity and yet retained normal levels of attachment to cell surface heparan sulfate and binding to nectin-1 receptor. Low-pH-triggered conformational changes in gB reported to date are reversible, despite irreversible low-pH inactivation. gB conformational changes and their reversibility were measured by antigenic analysis with a panel of monoclonal antibodies and by detecting changes in oligomeric conformation. Three-hour treatment of HSV-1 virions with pH 5 or multiple sequential treatments at pH 5 followed by neutral pH caused an irreversible >2.5 log infectivity reduction. While changes in several gB antigenic sites were reversible, alteration of the H126 epitope was irreversible. gB oligomeric conformational change remained reversible under all conditions tested. Altogether, our results reveal that oligomeric alterations and fusion domain changes represent distinct conformational changes in gB, and the latter correlates with irreversible low-pH inactivation of HSV. We propose that conformational change in the gB fusion domain is important for activation of membrane fusion during viral entry and that in the absence of a host target membrane, this change results in irreversible inactivation of virions.IMPORTANCE HSV-1 is an important pathogen with a high seroprevalence throughout the human population. HSV infects cells via multiple pathways, including a low-pH route into epithelial cells, the primary portal into the host. HSV is inactivated by low-pH preexposure, and gB, a class III fusion protein, undergoes reversible conformational changes in response to low-pH exposure. Here, we show that low-pH inactivation of HSV is irreversible and due to a defect in virion fusion activity. We identified an irreversible change in the fusion domain of gB following multiple sequential low-pH exposures or following prolonged low-pH treatment. This change appears to be separable from the alteration in gB quaternary structure. Together, the results are consistent with a model by which low pH can have an activating or inactivating effect on HSV depending on the presence of a target membrane.

Pseudorabies virus infection inhibits autophagy in permissive cells in vitro

A large number of studies have demonstrated that autophagy is involved in the infection processes of different pathogens. Autophagy is now recognized as an essential component of innate and adaptive immunity. Several herpesviruses have developed various strategies to evade this antiviral mechanism. Pseudorabies virus (PRV) is a swine herpesvirus with a broad host range that causes devastating disease in infected pigs. In this study, we described the interaction between PRV and autophagy for the first time. PRV infection had a dual effect on the cell autophagy response; during the early period of infection, PRV virions induced autophagy without viral replication, and with viral protein expression, PRV reduced the basal level of autophagy in several permissive cells. We observed that inhibit the level of autophagy could increase the titer of infectious PRV. We also found that the conserved alphaherpesvirus US3 tegument protein may reduce the level of autophagy via activation of the AKT/mTOR pathways in PRV infected cells. These findings suggest that autophagy likely contributes to clearance of PRV, and that the virus has evolved strategies to antagonize this pathway.

Brain transcriptomes of harbor seals demonstrate gene expression patterns of animals undergoing a metabolic disease and a viral infection

Diseases of marine mammals can be difficult to diagnose because of their life history and protected status. Stranded marine mammals have been a particularly useful resource to discover and comprehend the diseases that plague these top predators. Additionally, advancements in high-throughput sequencing (HTS) has contributed to the discovery of novel pathogens in marine mammals. In this study, we use a combination of HTS and stranded harbor seals (Phoca vitulina) to better understand a known and unknown brain disease. To do this, we used transcriptomics to evaluate brain tissues from seven neonatal harbor seals that expired from an unknown cause of death (UCD) and compared them to four neonatal harbor seals that had confirmed phocine herpesvirus (PhV-1) infections in the brain. Comparing the two disease states we found that UCD animals showed a significant abundance of fatty acid metabolic transcripts in their brain tissue, thus we speculate that a fatty acid metabolic dysregulation contributed to the death of these animals. Furthermore, we were able to describe the response of four young harbor seals with PhV-1 infections in the brain. PhV-1 infected animals showed a significant ability to mount an innate and adaptive immune response, especially to combat viral infections. Our data also suggests that PhV-1 can hijack host pathways for DNA packaging and exocytosis. This is the first study to use transcriptomics in marine mammals to understand host and viral interactions and assess the death of stranded marine mammals with an unknown disease. Furthermore, we show the value of applying transcriptomics on stranded marine mammals for disease characterization.

Herpesvirus Entry into Host Cells Mediated by Endosomal Low pH

Herpesviral pathogenesis stems from infection of multiple cell types including the site of latency and cells that support lytic replication. Herpesviruses utilize distinct cellular pathways, including low pH endocytic pathways, to enter different pathophysiologically relevant target cells. This review details the impact of the mildly acidic milieu of endosomes on the entry of herpesviruses, with particular emphasis on herpes simplex virus 1 (HSV-1). Epithelial cells, the portal of primary HSV-1 infection, support entry via low pH endocytosis mechanisms. Mildly acidic pH triggers reversible conformational changes in the HSV-1 class III fusion protein glycoprotein B (gB). In vitro treatment of herpes simplex virions with a similar pH range inactivates infectivity, likely by prematurely activating the viral entry machinery in the absence of a target membrane. How a given herpesvirus mediates both low pH and pH-independent entry events is a key unresolved question.

Have NEC Coat, Will Travel: Structural Basis of Membrane Budding During Nuclear Egress in Herpesviruses

Herpesviruses are unusual among enveloped viruses because they bud twice yet acquire a single envelope. Furthermore, unlike other DNA viruses that replicate in the nucleus, herpesviruses do not exit it by passing through the nuclear pores or by rupturing the nuclear envelope. Instead, herpesviruses have a complex mechanism of nuclear escape whereby nascent capsids bud at the inner nuclear membrane to form perinuclear virions that subsequently fuse with the outer nuclear membrane, releasing capsids into the cytosol. This makes them some of the very few known viruses that bud into the nuclear envelope. The envelope acquired during nuclear budding does not end up in the mature viral particle but instead allows the capsid to translocate from the nucleus into the cytosol. The viral nuclear egress complex (NEC) is a critical player in the nuclear egress, yet its function and mechanism have remained enigmatic. Recent studies have demonstrated that the NEC buds membranes without the help of other proteins by forming a honeycomb coat, which established the NEC as the first virally encoded budding machine that operates at the nuclear, as opposed to cytoplasmic, membrane. This review discusses our current understanding of the NEC budding mechanism, with the emphasis on studies that illuminated the structure of the NEC coat and its role in capsid budding during herpesvirus nuclear escape.

Exocytosis of Alphaherpesvirus Virions, Light Particles, and Glycoproteins Uses Constitutive Secretory Mechanisms

Many molecular and cell biological details of the alphaherpesvirus assembly and egress pathway remain unclear. Recently we developed a live-cell fluorescence microscopy assay of pseudorabies virus (PRV) exocytosis, based on total internal reflection fluorescence (TIRF) microscopy and a virus-encoded pH-sensitive fluorescent probe. Here, we use this assay to distinguish three classes of viral exocytosis in a nonpolarized cell type: (i) trafficking of viral glycoproteins to the plasma membrane, (ii) exocytosis of viral light particles, and (iii) exocytosis of virions. We find that viral glycoproteins traffic to the cell surface in association with constitutive secretory Rab GTPases and exhibit free diffusion into the plasma membrane after exocytosis. Similarly, both virions and light particles use these same constitutive secretory mechanisms for egress from infected cells. Furthermore, we show that viral light particles are distinct from cellular exosomes. Together, these observations shed light on viral glycoprotein trafficking steps that precede virus particle assembly and reinforce the idea that virions and light particles share a biogenesis and trafficking pathway.

The alphaherpesviruses, including the important human pathogens herpes simplex virus 1 (HSV-1), HSV-2, and varicella-zoster virus (VZV), are among the few viruses that have evolved to exploit the mammalian nervous system. These viruses typically cause mild recurrent herpetic or zosteriform lesions but can also cause debilitating herpes encephalitis, more frequently in very young, old, immunocompromised, or nonnatural hosts. Importantly, many of the molecular and cellular mechanisms of viral assembly and egress remain unclear. This study addresses the trafficking of viral glycoproteins to the plasma membrane, exocytosis of light particles, and exocytosis of virions. Trafficking of glycoproteins affects immune evasion and pathogenesis and may precede virus particle assembly. The release of light particles may also contribute to immune evasion and pathogenesis. Finally, exocytosis of virions is important to understand, as this final step in the virus replication cycle produces infectious extracellular particles capable of spreading to the next round of host cells.

3D printed nervous system on a chip

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

The diffusive way out: Herpesviruses remodel the host nucleus, enabling capsids to access the inner nuclear membrane

Herpesviruses are large DNA viruses that utilize the host nucleus for genome replication as well as capsid assembly. After maturation, these 125 nm large capsid assemblies must cross the nucleoplasm to engage the nuclear envelope and bud into the cytoplasm. Here we summarize our recent findings how this motility is facilitated. We suggest that herpesvirus induced nuclear remodeling allows capsids to move by diffusion in the nucleus and not by motor-dependent transport.

Simultaneous Visualization of Parental and Progeny Viruses by a Capsid-Specific HaloTag Labeling Strategy

Real-time, long-term, single-particle tracking (SPT) provides us an opportunity to explore the fate of individual viruses toward understanding the mechanisms underlying virus infection, which in turn could lead to the development of therapeutics against viral diseases. However, the research focusing on the virus assembly and egress by SPT remains a challenge because established labeling strategies could neither specifically label progeny viruses nor make them distinguishable from the parental viruses. Herein, we have established a temporally controllable capsid-specific HaloTag labeling strategy based on reverse genetic technology. VP26, the smallest pseudorabies virus (PrV) capsid protein, was fused with HaloTag protein and labeled with the HaloTag ligand during virus replication. The labeled replication-competent recombinant PrV harvested from medium can be applied directly in SPT experiments without further modification. Thus, virus infectivity, which is critical for the visualization and analysis of viral motion, is retained to the largest extent. Moreover, progeny viruses can be distinguished from parental viruses using diverse HaloTag ligands. Consequently, the entire course of virus infection and replication can be visualized continuously, including virus attachment and capsid entry, transportation of capsids to the nucleus along microtubules, docking of capsids on the nucleus, endonuclear assembly of progeny capsids, and the egress of progeny viruses. In combination with SPT, the established strategy represents a versatile means to reveal the mechanisms and dynamic global picture of the life cycle of a virus.

Open LED Illuminator: A Simple and Inexpensive LED Illuminator for Fast Multicolor Particle Tracking in Neurons

Dual-color live cell fluorescence microscopy of fast intracellular trafficking processes, such as axonal transport, requires rapid switching of illumination channels. Typical broad-spectrum sources necessitate the use of mechanical filter switching, which introduces delays between acquisition of different fluorescence channels, impeding the interpretation and quantification of highly dynamic processes. Light Emitting Diodes (LEDs), however, allow modulation of excitation light in microseconds. Here we provide a step-by-step protocol to enable any scientist to build a research-grade LED illuminator for live cell microscopy, even without prior experience with electronics or optics. We quantify and compare components, discuss our design considerations, and demonstrate the performance of our LED illuminator by imaging axonal transport of herpes virus particles with high temporal resolution.

Fluorescent Protein Approaches in Alpha Herpesvirus Research

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

Probing the interactions of CdTe quantum dots with pseudorabies virus

Quantum dots (QDs) have become one of the most promising luminescent materials for tracking viral infection in living cells. However, several issues regarding how QDs interact with the virus remain unresolved. Herein, the effects of Glutathione (GSH) capped CdTe QDs on virus were investigated by using pseudorabies virus (PRV) as a model. One-step growth curve and fluorescence colocalization analyses indicate that CdTe QDs inhibit PRV multiplication in the early stage of virus replication cycle by suppressing the invasion, but have no significant effect on the PRV penetration. Fluorescence spectrum analysis indicates that the size of QDs is reduced gradually after the addition of PRV within 30 min. Release of Cd²⁺ was detected during the interaction of QDs and PRV, resulting in a decreased number of viruses which can infect cells. Further Raman spectra and Circular Dichroism (CD) spectroscopy analyses reveal that the structure of viral surface proteins is altered by CdTe QDs adsorbed on the virus surface, leading to the inhibition of virus replication. This study facilitates an in-depth understanding of the pathogenic mechanism of viruses and provides a basis for QDs-labeled virus research.

Tegument Assembly and Secondary Envelopment of Alphaherpesviruses

Alphaherpesviruses like herpes simplex virus are large DNA viruses characterized by their ability to establish lifelong latent infection in neurons. As for all herpesviruses, alphaherpesvirus virions contain a protein-rich layer called "tegument" that links the DNA-containing capsid to the glycoprotein-studded membrane envelope. Tegument proteins mediate a diverse range of functions during the virus lifecycle, including modulation of the host-cell environment immediately after entry, transport of virus capsids to the nucleus during infection, and wrapping of cytoplasmic capsids with membranes (secondary envelopment) during virion assembly. Eleven tegument proteins that are conserved across alphaherpesviruses have been implicated in the formation of the tegument layer or in secondary envelopment. Tegument is assembled via a dense network of interactions between tegument proteins, with the redundancy of these interactions making it challenging to determine the precise function of any specific tegument protein. However, recent studies have made great headway in defining the interactions between tegument proteins, conserved across alphaherpesviruses, which facilitate tegument assembly and secondary envelopment. We summarize these recent advances and review what remains to be learned about the molecular interactions required to assemble mature alphaherpesvirus virions following the release of capsids from infected cell nuclei.

Investigating the biology of alpha herpesviruses with MS-based proteomics

Viruses are intracellular parasites that can only replicate and spread in cells of susceptible hosts. Alpha herpesviruses (α-HVs) contain double-stranded DNA genomes of at least 120 kb, encoding for 70 or more genes. The viral genome is contained in an icosahedral capsid that is surrounded by a proteinaceous tegument layer and a lipid envelope. Infection starts in epithelial cells and spreads to the peripheral nervous system. In the natural host, α-HVs establish a chronic latent infection that can be reactivated and rarely spread to the CNS. In the nonnatural host, viral infection will in most cases spread to the CNS with often fatal outcome. The host response plays a crucial role in the outcome of viral infection. α-HVs do not encode all the genes required for viral replication and spread. They need a variety of host gene products including RNA polymerase, ribosomes, dynein, and kinesin. As a result, the infected cell is dramatically different from the uninfected cell revealing a complex and dynamic interplay of viral and host components required to complete the virus life cycle. In this review, we describe the pivotal contribution of MS-based proteomics studies over the past 15 years to understand the complicated life cycle and pathogenesis of four α-HV species from the alphaherpesvirinae subfamily: Herpes simplex virus-1, varicella zoster virus, pseudorabies virus and bovine herpes virus-1. We describe the viral proteome dynamics during host infection and the host proteomic response to counteract such pathogens.

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.

Repair of spinal cord injury with neuronal relays: From fetal grafts to neural stem cells

Spinal cord injury (SCI) disrupts the long axonal tracts of the spinal cord leading to devastating loss of function. Cell transplantation in the injured spinal cord has the potential to lead to recovery after SCI via a variety of mechanisms. One such strategy is the formation of neuronal relays between injured long tract axons and denervated neurons. The idea of creating a neuronal relay was first proposed over 25 years ago when fetal tissue was first successfully transplanted into the injured rodent spinal cord. Advances in labeling of grafted cells and the development of neural stem cell culturing techniques have improved the ability to create and refine such relays. Several recent studies have examined the ability to create a novel neuronal circuit between injured axons and denervated targets. This approach is an alternative to long-distance regeneration of damaged axons that may provide a meaningful degree of recovery without direct recreation of lost pathways. This brief review will examine the contribution of fetal grafting to current advances in neuronal grafting. Of particular interest will be the ability of transplanted neurons derived from fetal grafts, neural precursor cells and neural stem cells to reconnect long distance motor and sensory pathways of the injured spinal cord. This article is part of a Special Issue entitled SI: Spinal cord injury.