Evidence of a role for nonmuscle myosin II in herpes simplex virus type 1 egress

Duck plague virus tegument protein vp22 plays a key role in the secondary envelopment and cell-to-cell spread

Duck plague virus (DPV) is one of the major infectious and fatal diseases of geese, ducks, and other wild waterfowl. The DPV UL49 gene product VP22 is one of the most abundant tegument proteins. However, the role of the DPV VP22 is enigmatic to be clarified. In this study, we found deletion of the UL49 gene resulted in reduced viral growth curve and smaller plaque size in duck embryo fibroblast (DEF) cells, confirming that DPV VP22 is required for efficient viral growth in vitro. In addition, deletion of the UL49 gene inhibited the secondary envelopment of the virus, the release of viral particles, and the spread of viruses between cells. Our study signified the importance of VP22 for DPV secondary envelopment, release, cell-to-cell spread, and accumulation of viral RNA. These findings provide a basis for further study of the function of VP22 in DPV or other herpesviruses.

Microfluidic model for in vitro acute Toxoplasma gondii infection and transendothelial migration

The protozoan parasite Toxoplasma gondii (T. gondii) causes one of the most common human zoonotic diseases and infects approximately one-third of the global population. T. gondii infects nearly every cell type and causes severe symptoms in susceptible populations. In previous laboratory animal studies, T. gondii movement and transmission were not analyzed in real time. In a three-dimensional (3D) microfluidic assay, we successfully supported the complex lytic cycle of T. gondii in situ by generating a stable microvasculature. The physiology of the T. gondii-infected microvasculature was monitored in order to investigate the growth, paracellular and transcellular migration, and transmission of T. gondii, as well as the efficacy of T. gondii drugs.

Myosin motors on the pathway of viral infections

Molecular motors are microscopic machines that use energy from adenosine triphosphate (ATP) hydrolysis to generate movement. While kinesins and dynein are molecular motors associated with microtubule tracks, myosins bind to and move on actin filaments. Mammalian cells express several myosin motors. They power cellular processes such as endo- and exocytosis, intracellular trafficking, transcription, migration, and cytokinesis. As viruses navigate through cells, they may take advantage or be hindered by host components and machinery, including the cytoskeleton. This review delves into myosins' cell roles and compares them to their reported functions in viral infections. In most cases, the previously described myosin functions align with their reported role in viral infections, although not in all cases. This opens the possibility that knowledge obtained from studying myosins in viral infections might shed light on new physiological roles for myosins in cells. However, given the high number of myosins expressed and the variety of viruses investigated in the different studies, it is challenging to infer whether the interactions found are specific to a single virus or can be applied to other viruses with the same characteristics. We conclude that the participation of myosins in viral cycles is still a largely unexplored area, especially concerning unconventional myosins.

Nectin-1 and Non-muscle Myosin Heavy Chain-IIB: Major Mediators of Herpes Simplex Virus-1 Entry Into Corneal Nerves

Herpes Simplex Virus 1 (HSV-1) invades corneal nerves upon its infection of the cornea and then establishes latency in the trigeminal ganglion (TG). The latent virus in TG is often reactivated and travels back to the cornea, causing recurrent herpes simplex keratitis (HSK). The entry of HSV-1 into the corneal nerve is considered the initial step of infection resulting in HSV-1 latency and HSK recurrence. Several gD and gB receptors have been identified, including nectin-1, herpes virus entry medium (HVEM) and 3-O-sulfated heparan sulfate (3-OS-HS) as gD receptors, and non-muscle myosin heavy chain IIA (NMHC-IIA), NMHC-IIB and myelin-associated glycoprotein (MAG) as gB receptors. However, which receptors contribute to the entry of HSV-1 into corneal nerves are yet to be determined. This study observed that receptors nectin-1, HVEM, 3-OS-HS, NMHC-IIA, and NMHC-IIB, not MAG, were expressed in healthy corneal nerves. Further, we cultured TG neurons extracted from mice in vitro to screen for functional gD/gB receptors. Both in vitro siRNA knockdown and in vivo antibody blocking of either nectin-1 or NMHC-IIB reduced the entry and the replication of HSV-1 as shown by qPCR analysis and immunofluorescence measure, respectively. Also, we observed that the re-localization and the upregulation expression of NMHC-IIB after HSV-1 exposure were inhibited when gD receptor nectin-1 was knocked down. These data suggest that nectin-1 was the main gD receptor and NMHC-IIB was the main gB receptor in mediating HSV-1 entry and hold promise as therapeutic targets for resolving HSV-1 latency and HSK recurrence.

Understanding the key functions of Myosins in viral infection

Myosins, a class of actin-based motor proteins existing in almost any organism, are originally considered only involved in driving muscle contraction, reshaping actin cytoskeleton, and anchoring or transporting cargoes, including protein complexes, organelles, vesicles. However, accumulating evidence reveals that myosins also play vital roles in viral infection, depending on viral species and infection stages. This review systemically summarizes the described various myosins, the performed functions, and the involved mechanisms or molecular pathways during viral infection. Meanwhile, the existing issues are also discussed. Additionally, the important technologies or agents, including siRNA, gene editing, and myosin inhibitors, would facilitate dissecting the actions and mechanisms for described and undescribed myosins, which could be adopted to prevent or control viral infection are also characterized.

Herpes simplex virus-1 utilizes the host actin cytoskeleton for its release from axonal growth cones

Herpes simplex virus type 1 (HSV-1) has evolved mechanisms to exploit the host cytoskeleton during entry, replication and exit from cells. In this study, we determined the role of actin and the molecular motor proteins, myosin II and myosin V, in the transport and release of HSV-1 from axon termini, or growth cones. Using compartmentalized neuronal devices, we showed that inhibition of actin polymerization, but not actin branching, significantly reduced the release of HSV-1 from axons. Furthermore, we showed that inhibition of myosin V, but not myosin II, also significantly reduced the release of HSV-1 from axons. Using confocal and electron microscopy, we determined that viral components are transported along axons to growth cones, despite actin or myosin inhibition. Overall, our study supports the role of actin in virus release from axonal growth cones and suggests myosin V as a likely candidate involved in this process.

Herpes simplex virus type 1 (HSV-1) is a ubiquitous human pathogen causing cold sores and genital herpes. HSV-1 infects sensory neurons of the peripheral nervous system where it establishes a lifelong infection and cannot be cured. Reactivation is common, with the virus transported back along sensory nerves, forming new lesions, or is shed asymptomatically. Antiviral resistance is emerging to current antivirals that target viral replication, indicating the need to identify new targets for future treatment. The host cell cytoskeleton plays an important role during transport of the virus. HSV-1 is transported along axons via microtubules; however, how the virus is released from axon termini, where actin predominates, is unknown. Here we show that an intact actin cytoskeleton is required for efficient virus release from axon termini. Furthermore, we show that myosin V, an actin based molecular motor that drives transport, is essential in virus release from axon termini. Together, this study defines the mechanisms behind HSV-1 release from axon termini which will guide future directions in identifying possible therapeutic targets for HSV-1.

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.

"Non-Essential" Proteins of HSV-1 with Essential Roles In Vivo: A Comprehensive Review

Viruses encode for structural proteins that participate in virion formation and include capsid and envelope proteins. In addition, viruses encode for an array of non-structural accessory proteins important for replication, spread, and immune evasion in the host and are often linked to virus pathogenesis. Most virus accessory proteins are non-essential for growth in cell culture because of the simplicity of the infection barriers or because they have roles only during a state of the infection that does not exist in cell cultures (i.e., tissue-specific functions), or finally because host factors in cell culture can complement their absence. For these reasons, the study of most nonessential viral factors is more complex and requires development of suitable cell culture systems and in vivo models. Approximately half of the proteins encoded by the herpes simplex virus 1 (HSV-1) genome have been classified as non-essential. These proteins have essential roles in vivo in counteracting antiviral responses, facilitating the spread of the virus from the sites of initial infection to the peripheral nervous system, where it establishes lifelong reservoirs, virus pathogenesis, and other regulatory roles during infection. Understanding the functions of the non-essential proteins of herpesviruses is important to understand mechanisms of viral pathogenesis but also to harness properties of these viruses for therapeutic purposes. Here, we have provided a comprehensive summary of the functions of HSV-1 non-essential proteins.

Equid Herpesvirus-1 Exploits the Extracellular Matrix of Mononuclear Cells to Ensure Transport to Target Cells

Mononuclear cells are the first line of defense against microbial infection. Yet, several viruses have evolved different mechanisms to overcome host defenses to ensure their spread. Here, we show unique mechanisms of how equid herpesvirus-1 manipulates peripheral blood mononuclear cells (PBMC) to travel further in the body. (1) "PBMC-hitching": at the initial contact, herpesviruses lurk in the extracellular matrix (ECM) of PBMC without entering the cells. The virus exploits the components of the ECM to bind, transport, and then egress to infect other cells. (2) "Intracellular delivery": transendothelial migration is a physiological mechanism where mononuclear cells can transmigrate through the endothelial cells. The virus was intangible and probably did not interfere with such a mechanism where the infected PBMC can probably deliver the virus inside the endothelium. (3) "Classical-fusion": this process is well mastered by herpesviruses due to a set of envelope glycoproteins that facilitate cell-cell fusion and virus spread.

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.

Alphaherpesvirus Major Tegument Protein VP22: Its Precise Function in the Viral Life Cycle

Alphaherpesviruses are zoonotic pathogens that can cause a variety of diseases in humans and animals and severely damage health. Alphaherpesvirus infection is a slow and orderly process that can lie dormant for the lifetime of the host but may be reactivated when the immune system is compromised. All alphaherpesviruses feature a protein layer called the tegument that lies between the capsid and the envelope. Virus protein (VP) 22 is one of the most highly expressed tegument proteins; there are more than 2,000 copies of this protein in each viral particle. VP22 can interact with viral proteins, cellular proteins, and chromatin, and these interactions play important roles. This review summarizes the latest literature and discusses the roles of VP22 in viral gene transcription, protein synthesis, virion assembly, and viral cell-to-cell spread with the purpose of enhancing understanding of the life cycle of herpesviruses and other pathogens in host cells. The molecular interaction information herein provides important reference data.

Bridging the Gap: Virus Long-Distance Spread via Tunneling Nanotubes

Tunneling nanotubes (TNTs) are actin-based intercellular conduits that connect distant cells and allow intercellular transfer of molecular information, including genetic information, proteins, lipids, and even organelles. Besides providing a means of intercellular communication, TNTs may also be hijacked by pathogens, particularly viruses, to facilitate their spread. Viruses of many different families, including retroviruses, herpesviruses, orthomyxoviruses, and several others have been reported to trigger the formation of TNTs or TNT-like structures in infected cells and use these structures to efficiently spread to uninfected cells. In the current review, we give an overview of the information that is currently available on viruses and TNT-like structures, and we discuss some of the standing questions in this field.

Non-muscle Myosin II: Role in Microbial Infection and Its Potential as a Therapeutic Target

Currently, the major measures of preventing and controlling microbial infection are vaccinations and drugs. However, the appearance of drug resistance microbial mounts is main obstacle in current anti-microbial therapy. One of the most ubiquitous actin-binding proteins, non-muscle myosin II (NM II) plays a crucial role in a wide range of cellular physiological activities in mammals, including cell adhesion, migration, and division. Nowadays, growing evidence indicates that aberrant expression or activity of NM II can be detected in many diseases caused by microbes, including viruses and bacteria. Furthermore, an important role for NM II in the infection of some microbes is verified. Importantly, modulating the expression of NM II with small hairpin RNA (shRNA) or the activity of it by inhibitors can affect microbial-triggered phenotypes. Therefore, NM II holds the promise to be a potential target for inhibiting the infection of microbes and even treating microbial-triggered discords. In spite of these, a comprehensive view on the functions of NM II in microbial infection and the regulators which have an impact on the roles of NM II in this context, is still lacking. In this review, we summarize our current knowledge on the roles of NM II in microbial-triggered discords and provide broad insights into its regulators. In addition, the existing challenge of investigating the multiple roles of NM II in microbial infection and developing NM II inhibitors for treating these microbial-triggered discords, are also discussed.

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.

Cytoskeletons in the Closet-Subversion in Alphaherpesvirus Infections

Actin filaments, microtubules and intermediate filaments form the cytoskeleton of vertebrate cells. Involved in maintaining cell integrity and structure, facilitating cargo and vesicle transport, remodelling surface structures and motility, the cytoskeleton is necessary for the successful life of a cell. Because of the broad range of functions these filaments are involved in, they are common targets for viral pathogens, including the alphaherpesviruses. Human-tropic alphaherpesviruses are prevalent pathogens carried by more than half of the world’s population; comprising herpes simplex virus (types 1 and 2) and varicella-zoster virus, these viruses are characterised by their ability to establish latency in sensory neurons. This review will discuss the known mechanisms involved in subversion of and transport via the cytoskeleton during alphaherpesvirus infections, focusing on protein-protein interactions and pathways that have recently been identified. Studies on related alphaherpesviruses whose primary host is not human, along with comparisons to more distantly related beta and gammaherpesviruses, are also presented in this review. The need to decipher as-yet-unknown mechanisms exploited by viruses to hijack cytoskeletal components—to reveal the hidden cytoskeletons in the closet—will also be addressed.

Genital Herpes: Insights into Sexually Transmitted Infectious Disease

Etiology, transmission and protection: Herpes simplex virus-2 (HSV-2) is a leading cause of sexually transmitted infections with recurring manifestations throughout the lifetime of infected hosts. Currently no effective vaccines or prophylactics exist that provide complete protection or immunity from the virus, which is endemic throughout the world. Pathology/Symptomatology: Primary and recurrent infections result in lesions and inflammation around the genital area and the latter accounts for majority of genital herpes instances. Immunocompromised patients including neonates are susceptible to additional systemic infections including debilitating consequences of nervous system inflammation. Epidemiology, incidence and prevalence: More than 500 million people are infected worldwide and most reported cases involve the age groups between 16-40 years, which coincides with an increase in sexual activity among this age group. While these numbers are an estimate, the actual numbers may be underestimated as many people are asymptomatic or do not report the symptoms. Treatment and curability: Currently prescribed medications, mostly nucleoside analogs, only reduce the symptoms caused by an active infection, but do not eliminate the virus or reduce latency. Therefore, no cure exists against genital herpes and infected patients suffer from periodic recurrences of disease symptoms for their entire lives. Molecular mechanisms of infection: The last few decades have generated many new advances in our understanding of the mechanisms that drive HSV infection. The viral entry receptors such as nectin-1 and HVEM have been identified, cytoskeletal signaling and membrane structures such as filopodia have been directly implicated in viral entry, host motor proteins and their viral ligands have been shown to facilitate capsid transport and many host and HSV proteins have been identified that help with viral replication and pathogenesis. New understanding has emerged on the role of autophagy and other innate immune mechanisms that are subverted to enhance HSV pathogenesis. This review summarizes our current understanding of HSV-2 and associated diseases and available or upcoming new treatments.

A VP22-Null HSV-1 Is Impaired in Inhibiting CD1d-Mediated Antigen Presentation

CD1d-restricted T (natural killer T [NKT]) cells are important for controlling a herpes simplex virus (HSV) infection. One of the mechanisms of immune evasion by HSV is to downregulate CD1d-mediated activation of NKT cells. VP22 is an HSV-1-encoded protein responsible for reorganizing the host cell's cytoskeletal network and viral spreading. We have previously shown that modification of the cytoskeleton can alter CD1d-mediated antigen presentation. In this study, we found that an HSV-1 lacking VP22 (ΔUL49) was impaired in its ability to inhibit CD1d-mediated antigen presentation compared with the wild-type (WT) virus; this was reversed by a repair virus (UL49R) in CD1d-expressing cells. We further demonstrated that CD1d recycling was inhibited by infection with WT and UL49R, but not the ΔUL49 virus. Ectopic expression of VP22 in CD1d-expressing cells complemented the VP22-deficient virus in inhibiting antigen presentation. Moreover, inhibiting viral protein synthesis rescued VP22-dependent inhibition of CD1d antigen presentation. In conclusion, our findings suggest that VP22 is required (but not sufficient) for the inhibition of CD1d-mediated antigen presentation by an HSV-1 infection.

Evidence for the Use of Multiple Mechanisms by Herpes Simplex Virus-1 R7020 to Inhibit Intimal Hyperplasia

Intimal hyperplasia (IH) is the primary cause of vein bypass graft failure. The smooth muscle cell (SMC) is a key element of IH as it phenotypically switches from a contractile to a synthetic state which can become pathological. R7020, which is an engineered strain of Herpes Simplex Virus-1, inhibits IH in animal models. Although it has many characteristics which make it a strong candidate for use as a prophylactic agent how it inhibits IH is not well understood. The objective of this study was to identify modes of action used by R7020 to function in blood vessels that may also contribute to its inhibition of IH. The cytopathic effect of R7020 on SMCs was determined in vitro and in a rabbit IH model. In vitro assays with R7020 infected SMCs were used to quantify the effect of dose on the release kinetics of the virus as well as the effects of R7020 on cell viability and the adhesion of peripheral blood mononuclear cells (PBMCs) to SMCs in the absence and presence of tumor necrosis factor alpha (TNF-α). The observed cytopathic effect, which included R7020 positive filopodia that extend from cell to cell and the formation of syncytia, suggests that R7020 remains cell associated after egress and spreads cell to cell instead of by diffusion through the extracellular fluid. This would allow the virus to rapidly infect vascular cells while evading the immune system. The directionality of the filopodia in vivo suggests that the virus preferentially travels from the media towards the intima targeting SMCs that would lead to IH. The formation of syncytia would inhibit SMC proliferation as incorporated cells are not able to multiply. It was also observed that R7020 induced the fusion of PBMCs with syncytia suggesting the virus may limit the effect of macrophages on IH. Furthermore, R7020 inhibited the proliferative effect of TNF-α, an inflammatory cytokine associated with increased IH. Thus, the results of this study suggest that R7020 inhibits IH through multiple mechanisms.

The contractome – a systems view of actomyosin contractility in non-muscle cells

Actomyosin contractility is a highly regulated process that affects many fundamental biological processes in each and every cell in our body. In this Cell Science at a Glance article and the accompanying poster, we mined the literature and databases to map the contractome of non-muscle cells. Actomyosin contractility is involved in at least 49 distinct cellular functions that range from providing cell architecture to signal transduction and nuclear activity. Containing over 100 scaffolding and regulatory proteins, the contractome forms a highly complex network with more than 230 direct interactions between its components, 86 of them involving phosphorylation. Mapping these interactions, we identify the key regulatory pathways involved in the assembly of actomyosin structures and in activating myosin to produce contractile forces within non-muscle cells at the exact time and place necessary for cellular function.

VP22 core domain from Herpes simplex virus 1 reveals a surprising structural conservation in both the Alpha- and Gammaherpesvirinae subfamilies

The viral tegument is a layer of proteins between the herpesvirus capsid and its outer envelope. According to phylogenetic studies, only a third of these proteins are conserved amongst the three subfamilies (Alpha-, Beta- and Gammaherpesvirinae) of the family Herpesviridae. Although some of these tegument proteins have been studied in more detail, the structure and function of the majority of them are still poorly characterized. VP22 from Herpes simplex virus 1 (subfamily Alphaherpesvirinae) is a highly interacting tegument protein that has been associated with tegument assembly. We have determined the crystal structure of the conserved core domain of VP22, which reveals an elongated dimer with several potential protein-protein interaction regions and a peptide-binding site. The structure provides us with the structural basics to understand the numerous functional mutagenesis studies of VP22 found in the literature. It also establishes an unexpected structural homology to the tegument protein ORF52 from Murid herpesvirus 68 (subfamily Gammaherpesvirinae). Homologues for both VP22 and ORF52 have been identified in their respective subfamilies. Although there is no obvious sequence overlap in the two subfamilies, this structural conservation provides compelling structural evidence for shared ancestry and functional conservation.

Nonmuscle myosin heavy chain IIb mediates herpes simplex virus 1 entry

Nonmuscle myosin heavy chain IIA (NMHC-IIA) has been reported to function as a herpes simplex virus 1 (HSV-1) entry coreceptor by interacting with viral envelope glycoprotein B (gB). Vertebrates have three genetically distinct isoforms of the NMHC-II, designated NMHC-IIA, NMHC-IIB, and NMHC-IIC. COS cells, which are readily infected by HSV-1, do not express NMHC-IIA but do express NMHC-IIB. This observation prompted us to investigate whether NMHC-IIB might associate with HSV-1 gB and be involved in an HSV-1 entry like NMHC-IIA. In these studies, we show that (i) NMHC-IIB coprecipitated with gB in COS-1 cells upon HSV-1 entry; (ii) a specific inhibitor of myosin light chain kinase inhibited cell surface expression of NMHC-IIB in COS-1 cells upon HSV-1 entry as well as HSV-1 infection, as reported with NMHC-IIA; (iii) overexpression of mouse NMHC-IIB in IC21 cells significantly increased their susceptibility to HSV-1 infection; and (iv) knockdown of NMHC-IIB in COS-1 cells inhibited HSV-1 infection as well as cell-cell fusion mediated by HSV-1 envelope glycoproteins. These results supported the hypothesis that, like NMHC-IIA, NMHC-IIB associated with HSV-1 gB and mediated HSV-1 entry.

IMPORTANCE

Herpes simplex virus 1 (HSV-1) was reported to utilize nonmuscle myosin heavy chain IIA (NMHC-IIA) as an entry coreceptor associating with gB. Vertebrates have three genetically distinct isoforms of NMHC-II. In these isoforms, NMHC-IIB is of special interest since it highly expresses in neuronal tissue, one of the most important cellular targets of HSV-1 in vivo. In this study, we demonstrated that the ability to mediate HSV-1 entry appeared to be conserved in NMHC-II isoforms. These results may provide an insight into the mechanism by which HSV-1 infects a wide variety of cell types in vivo.

Three-Dimensional Imaging of Viral Infections

Three-dimensional (3D) imaging technologies are beginning to have significant impact in the field of virology, as they are helping us understand how viruses take control of cells. In this article we review several methodologies for 3D imaging of cells and show how these technologies are contributing to the study of viral infections and the characterization of specialized structures formed in virus-infected cells. We include 3D reconstruction by transmission electron microscopy (TEM) using serial sections, electron tomography, and focused ion beam scanning electron microscopy (FIB-SEM). We summarize from these methods selected contributions to our understanding of viral entry, replication, morphogenesis, egress and propagation, and changes in the spatial architecture of virus-infected cells. In combination with live-cell imaging, correlative microscopy, and new techniques for molecular mapping in situ, the availability of these methods for 3D imaging is expected to provide deeper insights into understanding the structural and dynamic aspects of viral infection.

Life without double-headed non-muscle myosin II motor proteins

Non-muscle myosin II motor proteins (myosin IIA, myosin IIB, and myosin IIC) belong to a class of molecular motor proteins that are known to transduce cellular free-energy into biological work more efficiently than man-made combustion engines. Nature has given a single myosin II motor protein for lower eukaryotes and multiple for mammals but none for plants in order to provide impetus for their life. These specialized nanomachines drive cellular activities necessary for embryogenesis, organogenesis, and immunity. However, these multifunctional myosin II motor proteins are believed to go awry due to unknown reasons and contribute for the onset and progression of many autosomal-dominant disorders, cataract, deafness, infertility, cancer, kidney, neuronal, and inflammatory diseases. Many pathogens like HIV, Dengue, hepatitis C, and Lymphoma viruses as well as Salmonella and Mycobacteria are now known to take hostage of these dedicated myosin II motor proteins for their efficient pathogenesis. Even after four decades since their discovery, we still have a limited knowledge of how these motor proteins drive cell migration and cytokinesis. We need to enrich our current knowledge on these fundamental cellular processes and develop novel therapeutic strategies to fix mutated myosin II motor proteins in pathological conditions. This is the time to think how to relieve the hijacked myosins from pathogens in order to provide a renewed impetus for patients' life. Understanding how to steer these molecular motors in proliferating and differentiating stem cells will improve stem cell based-therapeutics development. Given the plethora of cellular activities non-muscle myosin motor proteins are involved in, their importance is apparent for human life.

Rab6 dependent post-Golgi trafficking of HSV1 envelope proteins to sites of virus envelopment

Herpes simplex virus 1 (HSV1) is an enveloped virus that uses undefined transport carriers for trafficking of its glycoproteins to envelopment sites. Screening of an siRNA library against 60 Rab GTPases revealed Rab6 as the principal Rab involved in HSV1 infection, with its depletion preventing Golgi-to-plasma membrane transport of HSV1 glycoproteins in a pathway used by several integral membrane proteins but not the luminal secreted protein Gaussia luciferase. Knockdown of Rab6 reduced virus yield to 1% and inhibited capsid envelopment, revealing glycoprotein exocytosis as a prerequisite for morphogenesis. Rab6-dependent virus production did not require the effectors myosin-II, bicaudal-D, dynactin-1 or rabkinesin-6, but was facilitated by ERC1, a factor involved in linking microtubules to the cell cortex. Tubulation and exocytosis of Rab6-positive, glycoprotein-containing membranes from the Golgi was substantially augmented by infection, resulting in enhanced and targeted delivery to cell tips. This reveals HSV1 morphogenesis as one of the first biological processes shown to be dependent on the exocytic activity of Rab6.

Myosin IIa activation is crucial in breast cancer derived galectin-1 mediated tolerogenic dendritic cell differentiation

BACKGROUND

Tolerogenic dendritic cells (tDCs) play important roles in immune tolerance, autoimmune disease, tissue transplantation, and the tumor micro-environment. Factors that induce tDCs have been reported, however the intracellular mechanisms involved are rarely discussed.

METHODS

Circulating CD14(+)CD16(+) of breast cancer patients and induced CD14(+)CD16(+) DCs were identified as tDCs by treating CD14(+) monocytes with galectin-1 and cancer cell-derived medium combined with IL-4 and GM-CSF. In addition, the 4T1 breast cancer syngeneic xenograft model was used to investigate the effect of galectin-1 in vivo.

RESULTS

The CD14(+)CD16(+) tDC population in the breast cancer patients was comparatively higher than that in the healthy donors, and both the MDA-MB-231 conditioned medium and galectin-1 could induce tDC differentiation. In a BALB/c animal model, the 4T1 breast cancer cell line enhanced IL-10 expression in CD11c(+) DCs which was down-regulated after knocking down the galectin-1 expression of 4T1 cells. Analysis of galectin-1 interacting proteins showed that myosin IIa was a major target of galectin-1 after internalization through a caveolin-dependent endocytosis. Myosin IIa specific inhibitor could diminish the effects of galectin-1 on monocyte-derived tDCs and also block the 4T1 cell induced CD11c(+)/Ly6G(+)/IL-10(+) in the BALB/c mice.

CONCLUSIONS

Galectin-1 can induce tDCs after internalizing into CD14(+) monocytes through the caveolae-dependent pathway and activating myosin IIa. For the breast cancer patients with a high galectin-1 expression, blebbistatin and genistein show potential in immune modulation and cancer immunotherapy.

GENERAL SIGNIFICANCE

Myosin IIa activation and galectin-1 endocytosis are important in tumor associated tDC development.

Viral and Cellular Components of AAV2 Replication Compartments

Adeno-associated virus 2 (AAV2) is a helpervirus-dependent parvovirus with a bi-phasic life cycle comprising latency in absence and lytic replication in presence of a helpervirus, such as adenovirus (Ad) or herpes simplex virus type 1 (HSV-1). Helpervirus-supported AAV2 replication takes place in replication compartments (RCs) in the cell nucleus where virus DNA replication and transcription occur. RCs consist of a defined set of helper virus-, AAV2-, and cellular proteins. Here we compare the profile of cellular proteins recruited into AAV2 RCs or identified in Rep78-associated complexes when either Ad or HSV-1 is the helpervirus, and we discuss the potential roles of some of these proteins in AAV2 and helpervirus infection.

HIV trafficking in host cells: motors wanted!

Throughout the viral replication cycle, viral proteins, complexes, and particles need to be transported within host cells. These transport events are dependent on the host cell cytoskeleton and molecular motors. However, the mechanisms by which virus is trafficked along cytoskeleton filaments and how molecular motors are recruited and regulated to guarantee successful integration of the viral genome and production of new viruses has only recently begun to be understood. Recent studies on HIV have identified specific molecular motors involved in the trafficking of these viral particles. Here we review recent literature on the transport of HIV components in the cell, provide evidence for the identity and role of molecular motors in this process, and highlight how these trafficking events may be related to those occurring with other viruses.

Inhibition of myosin light chain kinase can be targeted for the development of new therapies against herpes simplex virus type-1 infection

BACKGROUND

Herpes simplex virus type-1 (HSV-1) is the leading cause of infectious blindness worldwide. Through a multistep process, HSV-1 enters into naturally susceptible human corneal epithelial (HCE) cells where it establishes an optimal environment for viral replication and spread. HSV-1 employment of cytoskeletal proteins, kinases, and cell signalling pathways is crucial for the entry process.

METHODS

Here we demonstrate that non-muscle myosin IIA (NM-IIA) and/or a myosin activating kinase, myosin light chain kinase (MLCK), can be targeted for the development of new and effective therapies against HSV-1. HCE cells were incubated with MLCK inhibitors ML-7 and ML-9 and NM-IIA inhibitor blebbistatin. Following the application of inhibitors, HSV-1 entry and spread to neighbouring HCE cells was evaluated.

RESULTS

Upon application of MLCK inhibitors ML-7 and ML-9 and NM-IIA inhibitor blebbistatin, HSV-1 entry into HCE cells was significantly decreased. Furthermore, dramatic impairment of glycoprotein-mediated membrane fusion was seen in cells treated with MLCK inhibitors, thus establishing a role for MLCK activation in cell-to-cell fusion and multinucleated syncytial cell formation. These results also indicate that the activation of motor protein NM-IIA by MLCK is crucial for cytoskeletal changes required for HSV-1 infection of corneal cells.

CONCLUSIONS

We provide new evidence that NM-IIA and MLCK can be used as effective antiviral targets against ocular herpes.

Chlamydia trachomatis inclusion membrane protein CT228 recruits elements of the myosin phosphatase pathway to regulate release mechanisms

Chlamydia trachomatis replicates within a membrane-bound compartment termed an inclusion. The inclusion membrane is modified by the insertion of multiple proteins known as Incs. In a yeast two-hybrid screen, an interaction was found between the inclusion membrane protein CT228 and MYPT1, a subunit of myosin phosphatase. MYPT1 was recruited peripherally around the inclusion, whereas the phosphorylated, inactive form was localized to active Src-family kinase-rich microdomains. Phosphorylated myosin light chain 2 (MLC2), myosin light chain kinase (MLCK), myosin IIA, and myosin IIB also colocalized with inactive MYPT1. The role of these proteins was examined in the context of host-cell exit mechanisms (i.e., cell lysis and extrusion of intact inclusions). Inhibition of myosin II or small interfering RNA depletion of myosin IIA, myosin IIB, MLC2, or MLCK reduced chlamydial extrusion, thus favoring lytic events as the primary means of release. These studies provide insights into the regulation of egress mechanisms by C. trachomatis.

Rho-ROCK and Rac-PAK signaling pathways have opposing effects on the cell-to-cell spread of Marek's Disease Virus

Marek's Disease Virus (MDV) is an avian alpha-herpesvirus that only spreads from cell-to-cell in cell culture. While its cell-to-cell spread has been shown to be dependent on actin filament dynamics, the mechanisms regulating this spread remain largely unknown. Using a recombinant BAC20 virus expressing an EGFPVP22 tegument protein, we found that the actin cytoskeleton arrangements and cell-cell contacts differ in the center and periphery of MDV infection plaques, with cells in the latter areas showing stress fibers and rare cellular projections. Using specific inhibitors and activators, we determined that Rho-ROCK pathway, known to regulate stress fiber formation, and Rac-PAK, known to promote lamellipodia formation and destabilize stress fibers, had strong contrasting effects on MDV cell-to-cell spread in primary chicken embryo skin cells (CESCs). Inhibition of Rho and its ROCKs effectors led to reduced plaque sizes whereas inhibition of Rac or its group I-PAKs effectors had the adverse effect. Importantly, we observed that the shape of MDV plaques is related to the semi-ordered arrangement of the elongated cells, at the monolayer level in the vicinity of the plaques. Inhibition of Rho-ROCK signaling also resulted in a perturbation of the cell arrangement and a rounding of plaques. These opposing effects of Rho and Rac pathways in MDV cell-to-cell spread were validated for two parental MDV recombinant viruses with different ex vivo spread efficiencies. Finally, we demonstrated that Rho/Rac pathways have opposing effects on the accumulation of N-cadherin at cell-cell contact regions between CESCs, and defined these contacts as adherens junctions. Considering the importance of adherens junctions in HSV-1 cell-to-cell spread in some cell types, this result makes of adherens junctions maintenance one potential and attractive hypothesis to explain the Rho/Rac effects on MDV cell-to-cell spread. Our study provides the first evidence that MDV cell-to-cell spread is regulated by Rho/Rac signaling.

Cofilin 1-mediated biphasic F-actin dynamics of neuronal cells affect herpes simplex virus 1 infection and replication

Herpes simplex virus 1 (HSV-1) invades the nervous system and causes pathological changes. In this study, we defined the remodeling of F-actin and its possible mechanisms during HSV-1 infection of neuronal cells. HSV-1 infection enhanced the formation of F-actin-based structures in the early stage of infection, which was followed by a continuous decrease in F-actin during the later stages of infection. The disruption of F-actin dynamics by chemical inhibitors significantly reduced the efficiency of viral infection and intracellular HSV-1 replication. The active form of the actin-depolymerizing factor cofilin 1 was found to increase at an early stage of infection and then to continuously decrease in a manner that corresponded to the remodeling pattern of F-actin, suggesting that cofilin 1 may be involved in the biphasic F-actin dynamics induced by HSV-1 infection. Knockdown of cofilin 1 impaired HSV-1-induced F-actin assembly during early infection and inhibited viral entry; however, overexpression of cofilin 1 did not affect F-actin assembly or viral entry during early infection but decreased intracellular viral reproduction efficiently. Our results, for the first time, demonstrated the biphasic F-actin dynamics in HSV-1 neuronal infection and confirmed the association of F-actin with the changes in the expression and activity of cofilin 1. These results may provide insight into the mechanism by which HSV-1 productively infects neuronal cells and causes pathogenesis.

Nonmuscle myosin-2: mix and match

Members of the nonmuscle myosin-2 (NM-2) family of actin-based molecular motors catalyze the conversion of chemical energy into directed movement and force thereby acting as central regulatory components of the eukaryotic cytoskeleton. By cyclically interacting with adenosine triphosphate and F-actin, NM-2 isoforms promote cytoskeletal force generation in established cellular processes like cell migration, shape changes, adhesion dynamics, endo- and exo-cytosis, and cytokinesis. Novel functions of the NM-2 family members in autophagy and viral infection are emerging, making NM-2 isoforms regulators of nearly all cellular processes that require the spatiotemporal organization of cytoskeletal scaffolding. Here, we assess current views about the role of NM-2 isoforms in these activities including the tight regulation of NM-2 assembly and activation through phosphorylation and how NM-2-mediated changes in cytoskeletal dynamics and mechanics affect cell physiological functions in health and disease.

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.

Actin in herpesvirus infection

Actin is important for a variety of cellular processes, including uptake of extracellular material and intracellular transport. Several emerging lines of evidence indicate that herpesviruses exploit actin and actin-associated myosin motors for viral entry, intranuclear transport of capsids, and virion egress. The goal of this review is to explore these processes and to highlight potential future directions for this area of research.

Myosin Va enhances secretion of herpes simplex virus 1 virions and cell surface expression of viral glycoproteins

The final step in the egress of herpes simplex virus (HSV) virions requires virion-laden vesicles to bypass cortical actin and fuse with the plasma membrane, releasing virions into the extracellular space. Little is known about the host or viral proteins involved. In the current study, we noted that the conformation of myosin Va (myoVa), a protein known to be involved in melanosome and secretory granule trafficking to the plasma membrane in melanocytes and neuroendocrine cells, respectively, was altered by 4 h after infection with HSV-1 such that an N-terminal epitope expected to be masked in its inactive state was rendered immunoreactive. Wild-type myoVa localized throughout the cytoplasm and to a limited extent in the nuclei of HSV-infected cells. Two different dominant negative myoVa molecules containing cargo-binding domains but lacking the lever arms and actin-binding domains colocalized with markers of the trans-Golgi network (TGN). Expression of dominant negative myoVa isoforms reduced secretion of HSV-1 infectivity into the medium by 50 to 75%, reduced surface expression of glycoproteins B, M, and D, and increased intracellular virus infectivity to levels consistent with increased retention of virions in the cytoplasm. These data suggest that myoVa is activated during HSV-1 infection to help transport virion- and glycoprotein-laden vesicles from the TGN, through the cortical actin, to the plasma membrane. We cannot exclude a role for myoVa in promoting fusion of these vesicles with the inner surface of the plasma membrane. These data also indicate that myoVa is involved in exocytosis in human epithelial cells as well as other cell types.

Conventional myosins - unconventional functions

While the discovery of unconventional myosins raised expectations that their actions were responsible for most aspects of actin-based cell motility, few anticipated the wide range of cellular functions that would remain the purview of conventional two-headed myosins. The three nonsarcomeric, cellular myosins-M2A, M2B and M2C-participate in diverse roles including, but not limited to: neuronal dynamics, axon guidance and synaptic transmission; endothelial cell migration; cell adhesion, polarity, fusion and cytokinesis; vesicle trafficking and viral egress. These three conventional myosins each take on specific, differing functional roles during development and maturity, characteristic of each cell lineage; exact roles depend on the developmental stage of the cell, cellular location, upstream regulatory controls, relative isoform expression, orientation and associated state of the actin cytoscaffolds in which these myosins operate. Here, we discuss the separate yet related roles that characterise the actions of M2A, M2B and M2C in various cell types and show that these conventional myosins are responsible for functions as unconventional as any performed by unconventional myosins.

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.

Intracellular trafficking of VP22 in bovine herpesvirus-1 infected cells

The intracellular trafficking of different VP22-enhanced yellow fluorescent protein (EYFP) fusion proteins expressed by bovine herpesvirus-1 (BHV-1) recombinants was examined by live-cell imaging. Our results demonstrate that (i) the fusion of EYFP to the C terminus of VP22 does not alter the trafficking of the protein in infected cells, (ii) VP22 expressed during BHV-1 infection translocates to the nucleus through three different pathways, namely early mitosis-dependent nuclear translocation, late massive nuclear translocation that follows a prolonged cytoplasmic stage of the protein in non-mitotic cells, and accumulation of a small subset of VP22 in discrete dot-like nuclear domains during its early cytoplasmic stage, (iii) the addition of the SV40 large-T-antigen nuclear localization signal (NLS) to VP22-EYFP abrogates its early cytoplasmic stage, and (iv) the VP22 (131)PRPR(134) NLS is not required for the late massive nuclear translocation of the protein, but this motif is essential for the targeting of VP22 to discrete dot-like nuclear domains during the early cytoplasmic stage. These results show that the amount of VP22 in the nucleus is precisely regulated at different stages of BHV-1 infection and suggest that the early pathways of VP22 nuclear accumulation may be more relevant to the infection process as the late massive nuclear influx starts when most of the viral progeny has already emerged from the cell.

Role of viral chromatin structure in the regulation of herpes simplex virus 1 gene expression and replication

Herpes simplex virus 1 initially infects epithelial cells during the lytic phase of its infectious cycle, followed by establishment of the latent phase within neuronal cells. The two different phases of infection are characterized by distinct gene-expression profiles, involving a temporal gene-expression pattern during the lytic phase succeeded by a complete shutdown of all gene expression, except for one abundant transcript, during the latent phase. The mechanisms controlling these varying degrees of gene expression appear to involve regulation of the viral chromatin structure, presumably using many of the same tactics employed by the host cell.

Epstein-Barr virus BDLF2-BMRF2 complex affects cellular morphology

Herpesvirus glycoproteins often form specific heterodimers that can fulfil functions that cannot be carried out by either of the partners acting alone. This study showed that interactions between the Epstein-Barr virus (EBV) multi-spanning transmembrane envelope protein BMRF2 and type II membrane protein BDLF2 influence the way in which these proteins are trafficked in the cell, and hence the subcellular compartment in which they accumulate. When expressed transiently in mammalian cells, BDLF2 accumulated in the endoplasmic reticulum (ER), whereas BMRF2 accumulated in the ER and Golgi apparatus. However, when the two proteins were co-expressed, BDLF2 was transported with BMRF2 to the Golgi apparatus and from there to the plasma membrane, where the proteins co-localized extensively. The distribution of the two proteins at the plasma membrane was reproducibly associated with dramatic changes in cellular morphology, including the formation of enlarged membrane protrusions and cellular processes whose adhesion extremities were organized by the actin cytoskeleton. A dominant-active form of the small GTPase RhoA was epistatic to this morphological phenotype, suggesting that RhoA is a central component of the signalling pathway that reorganizes the cytoskeleton in response to BDLF2-BMRF2. It was concluded that EBV produces a glycoprotein heterodimer that induces changes in cellular morphology through reorganization of the actin cytoskeleton and may facilitate virion spread between cells.

Avoiding the void: cell-to-cell spread of human viruses

The initial stages of animal virus infection are generally described as the binding of free virions to permissive target cells followed by entry and replication. Although this route of infection is undoubtedly important, many viruses that are pathogenic for humans, including HIV-1, herpes simplex virus and measles, can also move between cells without diffusing through the extracellular environment. Cell-to-cell spread not only facilitates rapid viral dissemination, but may also promote immune evasion and influence disease. This Review discusses the various mechanisms by which viruses move directly between cells and the implications of this for viral dissemination and pathogenesis.

Actin and Rho GTPases in herpesvirus biology

Viruses have evolved a variety of interactions with host cells to create an optimal niche for viral replication, persistence and spread. The actin cytoskeleton of the host cell and actin-regulating Rho GTPase signaling pathways can be involved in several of these interactions. This review focuses on recent findings on herpesvirus interactions with actin and Rho GTPases during viral entry, replication in the nucleus and egress. Unraveling these often fascinating interactions might also provide additional insights into sometimes poorly known aspects of actin biology (e.g. its role in the nucleus) and in the development of novel antiviral therapies.

The pseudorabies virus VP1/2 tegument protein is required for intracellular capsid transport

Transport of capsids in cells is critical to alphaherpesvirus infection and pathogenesis; however, viral factors required for transport have yet to be identified. Here we provide a detailed examination of capsid dynamics during the egress phase of infection in Vero cells infected with pseudorabies virus. We demonstrate that the VP1/2 tegument protein is required for processive microtubule-based transport of capsids in the cytoplasm. A second tegument protein that binds to VP1/2, UL37, was necessary for wild-type transport but was not essential for this process. Both proteins were also required for efficient nuclear egress of capsids to the cytoplasm.

Budding of Marburgvirus is associated with filopodia

Viruses exploit the cytoskeleton of host cells to transport their components and spread to neighbouring cells. Here we show that the actin cytoskeleton is involved in the release of Marburgvirus (MARV) particles. We found that peripherally located nucleocapsids and envelope precursors of MARV are located either at the tip or at the side of filopodial actin bundles. Importantly, viral budding was almost exclusively detected at filopodia. Inhibiting actin polymerization in MARV-infected cells significantly diminished the amount of viral particles released into the medium. This suggested that dynamic polymerization of actin in filopodia is essential for efficient release of MARV. The viral matrix protein VP40 plays a key role in the release of MARV particles and we found that the intracellular localization of recombinant VP40 and its release in form of virus-like particles were strongly influenced by overexpression or inhibition of myosin 10 and Cdc42, proteins important in filopodia formation and function. We suggest that VP40, which is capable of interacting with viral nucleocapsids, provides an interface of MARV subviral particles and filopodia. As filopodia are in close contact with neighbouring cells, usurpation of these structures may facilitate spread of MARV to adjacent cells.

Gene therapy progress and prospects: viral trafficking during infection

The intracellular steps involved in viral infection, namely cytoplasmic trafficking and nuclear import, are critical events in the viral life cycle that have lagged behind other areas of viral research. This review examines recent advances in our understanding of these steps for viruses commonly employed as viral gene delivery vectors. Steps governing the cytoplasmic trafficking and nuclear import of Herpes Simplex virus, Human Immunodeficiency virus and Adenovirus are reviewed in this article.