Function of dynein and dynactin in herpes simplex virus capsid transport

The expanding clinical and genetic spectrum of DYNC1H1-related disorders

Intracellular trafficking involves an intricate machinery of motor complexes, including the dynein complex, to shuttle cargo for autophagolysosomal degradation. Deficiency in dynein axonemal chains, as well as cytoplasmic light and intermediate chains, have been linked with ciliary dyskinesia and skeletal dysplasia. The cytoplasmic dynein 1 heavy chain protein (DYNC1H1) serves as a core complex for retrograde trafficking in neuronal axons. Dominant pathogenic variants in DYNC1H1 have been previously implicated in peripheral neuromuscular disorders (NMD) and neurodevelopmental disorders (NDD). As heavy-chain dynein is ubiquitously expressed, the apparent selectivity of heavy chain dyneinopathy for motor neuronal phenotypes remains currently unaccounted for. Here, we aimed to evaluate the full DYNC1H1-related clinical, molecular and imaging spectrum, including multisystem features and novel phenotypes presenting throughout life. We identified 47 cases from 43 families with pathogenic heterozygous variants in DYNC1H1 (aged 0-59 years) and collected phenotypic data via a comprehensive standardized survey and clinical follow-up appointments. Most patients presented with divergent and previously unrecognized neurological and multisystem features, leading to significant delays in genetic testing and establishing the correct diagnosis. Neurological phenotypes include novel autonomic features, previously rarely described behavioral disorders, movement disorders and periventricular lesions. Sensory neuropathy was identified in nine patients (median age of onset 10.6 years), of which five were only diagnosed after the second decade of life, and three had a progressive age-dependent sensory neuropathy. Novel multisystem features included primary immunodeficiency, bilateral sensorineural hearing loss, organ anomalies and skeletal manifestations, resembling the phenotypic spectrum of other dyneinopathies. We also identified an age-dependent biphasic disease course with developmental regression in the first decade and, following a period of stability, neurodegenerative progression after the second decade of life. Of note, we observed several cases in whom neurodegeneration appeared to be prompted by intercurrent systemic infections with double-stranded DNA viruses (Herpesviridae) or single-stranded RNA viruses (Ross River fever, SARS-CoV-2). Moreover, the disease course appeared to be exacerbated by viral infections regardless of age and/or severity of neurodevelopmental disorder manifestations, indicating a role of dynein in anti-viral immunity and neuronal health. In summary, our findings expand the clinical, imaging and molecular spectrum of pathogenic DYNC1H1 variants beyond motor neuropathy disorders and suggest a life-long continuum and age-related progression due to deficient intracellular trafficking. This study will facilitate early diagnosis and improve counselling and health surveillance of affected patients.

A Hitchhiker's Guide Through the Cell: The World According to the Capsids of Alphaherpesviruses

The nucleoplasm, the cytosol, the inside of virions, and again the cytosol comprise the world in which the capsids of alphaherpesviruses encounter viral and host proteins that support or limit them in performing their tasks. Here, we review the fascinating conundrum of how specific protein-protein interactions late in alphaherpesvirus infection orchestrate capsid nuclear assembly, nuclear egress, and cytoplasmic envelopment, but target incoming capsids to the nuclear pores in naive cells to inject the viral genomes into the nucleoplasm for viral transcription and replication. Multiple capsid interactions with viral and host proteins have been characterized using viral mutants and assays that reconstitute key stages of the infection cycle. Keratinocytes, fibroblasts, mucosal epithelial cells, neurons, and immune cells employ cell type-specific intrinsic and cytokine-induced resistance mechanisms to restrict several stages of the viral infection cycle. However, concomitantly, alphaherpesviruses have evolved countermeasures to ensure efficient capsid function during infection.

A review of HSV pathogenesis, vaccine development, and advanced applications

Herpes simplex virus (HSV), an epidemic human pathogen threatening global public health, gains notoriety for its complex pathogenesis that encompasses lytic infection of mucosal cells, latent infection within neurons, and periodic reactivation. This intricate interplay, coupled with HSV's sophisticated immune evasion strategies, gives rise to various diseases, including genital lesions, neonatal encephalitis, and cancer. Despite more than 70 years of relentless research, an effective preventive or therapeutic vaccine against HSV has yet to emerge, primarily due to the limited understanding of virus-host interactions, which in turn impedes the identification of effective vaccine targets. However, HSV's unique pathological features, including its substantial genetic load capacity, high replicability, transmissibility, and neurotropism, render it a promising candidate for various applications, spanning oncolytic virotherapy, gene and immune therapies, and even as an imaging tracer in neuroscience. In this review, we comprehensively update recent breakthroughs in HSV pathogenesis and immune evasion, critically summarize the progress made in vaccine candidate development, and discuss the multifaceted applications of HSV as a biological tool. Importantly, we highlight both success and challenges, emphasizing the critical need for intensified research into HSV, with the aim of providing deeper insights that can not only advance HSV treatment strategies but also broaden its application horizons.

Functions of the UL51 protein during the herpesvirus life cycle

The herpesvirus UL51 protein is a multifunctional tegument protein involved in the regulation of multiple aspects of the viral life cycle. This article reviews the biological characteristics of the UL51 protein and its functions in herpesviruses, including participating in the maintenance of the viral assembly complex (cVAC) during viral assembly, affecting the production of mature viral particles and promoting primary and secondary envelopment, as well as its positive impact on viral cell-to-cell spread (CCS) through interactions with multiple viral proteins and its key role in the proliferation and pathogenicity of the virus in the later stage of infection. This paper discusses how the UL51 protein participates in the life cycle of herpesviruses and provides new ideas for further research on UL51 protein function.

Stabilizing microtubules aids neurite structure and disrupts syncytia formation in human cytomegalovirus-infected human forebrain neurons

Human cytomegalovirus (HCMV) is a prolific human herpesvirus that infects most individuals by adulthood. While typically asymptomatic in adults, congenital infection can induce serious neurological symptoms including hearing loss, visual deficits, cognitive impairment, and microcephaly in 10-15% of cases. HCMV has been shown to infect most neural cells with our group recently demonstrating this capacity in stem cell-derived forebrain neurons. Infection of neurons induces deleterious effects on calcium dynamics and electrophysiological function paired with gross restructuring of neuronal morphology. Here, we utilize an iPSC-derived model of the human forebrain to demonstrate how HCMV infection induces syncytia, drives neurite retraction, and remodels microtubule networks to promote viral production and release. We establish that HCMV downregulates microtubule associated proteins at 14 days postinfection while simultaneously sparing other cytoskeletal elements, and this includes HCMV-driven alterations to microtubule stability. Further, we pharmacologically modulate microtubule dynamics using paclitaxel (stabilize) and colchicine (destabilize) to examine the effects on neurite structure, syncytial morphology, assembly compartment formation, and viral release. With paclitaxel, we found improvement of neurite outgrowth with a corresponding disruption to HCMV-induced syncytia formation and Golgi network disruptions but with limited impact on viral titers. Together, these data suggest that HCMV infection-induced disruption of microtubules in human cortical neurons can be partially mitigated with microtubule stabilization, suggesting a potential avenue for future neuroprotective therapeutic exploration.

Herpes simplex virus type-1 cVAC formation in neuronal cells is mediated by dynein motor function and glycoprotein retrieval from the plasma membrane

Herpesvirus assembly requires the cytoplasmic association of large macromolecular and membrane structures that derive from both the nucleus and cytoplasmic membrane systems. Results from the study of human cytomegalovirus (HCMV) in cells where it organizes a perinuclear cytoplasmic virus assembly compartment (cVAC) show a clear requirement for the minus-end-directed microtubule motor, dynein, for virus assembly. In contrast, the assembly of herpes simplex virus -1 (HSV-1) in epithelial cells where it forms multiple dispersed, peripheral assembly sites is only mildly inhibited by the microtubule-depolymerizing agent, nocodazole. Here, we make use of a neuronal cell line system in which HSV-1 forms a single cVAC and show that dynein and its co-factor dynactin localize to the cVAC, and dynactin is associated with membranes that contain the virion tegument protein pUL11. We also show that the virus membrane-associated structural proteins pUL51 and the viral envelope glycoprotein gE arrive at the cVAC by different routes. Specifically, gE arrives at the cVAC after retrieval from the plasma membrane, suggesting the need for an intact retrograde transport system. Finally, we demonstrate that inhibition of dynactin function profoundly inhibits cVAC formation and virus production during the cytoplasmic assembly phase of infection.IMPORTANCEMany viruses reorganize cytoplasmic membrane systems and macromolecular transport systems to promote the production of progeny virions. Clarifying the mechanisms by which they accomplish this may reveal novel therapeutic strategies and illustrate mechanisms that are critical for normal cellular organization. Here, we explore the mechanism by which HSV-1 moves macromolecular and membrane cargo to generate a virus assembly compartment in the infected cell. We find that the virus makes use of a well-characterized, microtubule-based transport system that is stabilized against drugs that disrupt microtubules.

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

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

Nuclear RPSA senses viral nucleic acids to promote the innate inflammatory response

Innate sensors initiate the production of type I interferons (IFN-I) and proinflammatory cytokines to protect host from viral infection. Several innate nuclear sensors that mainly induce IFN-I production have been identified. Whether there exist innate nuclear sensors that mainly induce proinflammatory cytokine production remains to be determined. By functional screening, we identify 40 S ribosomal protein SA (RPSA) as a nuclear protein that recognizes viral nucleic acids and predominantly promotes proinflammatory cytokine gene expression in antiviral innate immunity. Myeloid-specific Rpsa-deficient mice exhibit less innate inflammatory response against infection with Herpes simplex virus-1 (HSV-1) and Influenza A virus (IAV), the viruses replicating in nucleus. Mechanistically, nucleus-localized RPSA is phosphorylated at Tyr204 upon infection, then recruits ISWI complex catalytic subunit SMARCA5 to increase chromatin accessibility of NF-κB to target gene promotors without affecting innate signaling. Our results add mechanistic insights to an intra-nuclear way of initiating proinflammatory cytokine expression in antiviral innate defense.

Genomic Markers Associated with Cytomegalovirus DNAemia in Kidney Transplant Recipients

Human cytomegalovirus (CMV) is a major pathogen after solid organ transplantation, leading to high morbidity and mortality. Transplantation from a CMV-seropositive donor to a CMV-seronegative recipient (D+/R-) is associated with high risk of CMV disease. However, that risk is not uniform, suggesting a role for host factors in immune control of CMV. To identify host genetic factors that control CMV DNAemia post transplantation, we performed a whole-exome association study in two cohorts of D+/R- kidney transplant recipients. Quantitative CMV DNA was measured for at least one year following transplantation. Several CMV-protective single-nucleotide polymorphisms (SNPs) were identified in the first cohort (72 patients) but were not reproducible in the second cohort (126 patients). A meta-analysis of both cohorts revealed several SNPs that were significantly associated with protection from CMV DNAemia. The copy number variation of several genes was significantly different between recipients with and without CMV DNAemia. Amongst patients with CMV DNAemia in the second cohort, several variants of interest (p < 5 × 10-5), the most common of which was NLRC5, were associated with peak viral load. We provide new predictive genetic markers for protection of CMV DNAemia. These markers should be validated in larger cohorts.

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

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

Smart design of universally decorated nanoparticles for drug delivery applications driven by active transport

Targeting the cell nucleus remains a challenge for drug delivery. Here, we present a universal platform for the smart design of nanoparticle (NP) decoration that is based on: (i) a spacer polymer, commonly biotin-polyethylene-glycol-thiol, whose grafting density and molecular weight can be tuned for optimized performance, and (ii) protein binding peptides, such as cell penetrating peptides (CPPs), cancer-targeting peptides, or nuclear localization signal (NLS) peptides, that are linked to the PEG free-end by universal chemistry. We manifested our platform with two different bromo-acetamide (Br-Ac) modified NLSs. We used cell extract-based and live cell assays to demonstrate the recruitment of dynein motor proteins, which drive the NP active transport toward the nucleus, and the enhancement of cellular and nuclear entry, manifesting the properties of NLS as a CPP. Our control of the NP decoration scheme, and the modularity of our platform, carry great advantages for nano-carrier design for drug delivery applications.

Microtubules and viral infection

Microtubules (MTs) form rapidly adaptable, complex intracellular networks of filaments that not only provide structural support, but also form the tracks along which motors traffic macromolecular cargos to specific sub-cellular sites. These dynamic arrays play a central role in regulating various cellular processes including cell shape and motility as well as cell division and polarization. Given their complex organization and functional importance, MT arrays are carefully controlled by many highly specialized proteins that regulate the nucleation of MT filaments at distinct sites, their dynamic growth and stability, and their engagement with other subcellular structures and cargoes destined for transport. This review focuses on recent advances in our understanding of how MTs and their regulatory proteins function, including their active targeting and exploitation, during infection by viruses that utilize a wide variety of replication strategies that occur within different cellular sub-compartments or regions of the cell.

A Baseline Cellular Antiviral State Is Maintained by cGAS and Its Most Frequent Naturally Occurring Variant rs610913

Upon recognition of aberrantly located DNA, the innate immune sensor cyclic GMP-AMP synthase (cGAS) activates stimulator of IFN genes (STING)/IFN regulatory factor (IRF)3-driven antiviral responses. In this study, we characterized the ability of a specific variant of the human cGAS-encoding gene MB21D1, rs610913, to alter cGAS-mediated DNA sensing and viral infection. rs610913 is a frequent G>T polymorphism resulting in a P261H exchange in the cGAS protein. Data from the International Collaboration for the Genomics of HIV suggested that rs610913 nominally associates with HIV-1 acquisition in vivo. Molecular modeling of cGAS(P261H) hinted toward the possibility for an additional binding site for a potential cellular cofactor in cGAS dimers. However, cGAS(wild-type [WT]) or cGAS(P261H)-reconstituted THP-1 cGAS knockout cells shared steady-state expression of IFN-stimulated genes, as opposed to cells expressing the enzymatically inactive cGAS(G212A/S213A). Accordingly, cGAS(WT) and cGAS(P261H) cells were less susceptible to lentiviral transduction and infection with HIV-1, HSV-1, and Chikungunya virus as compared with cGAS knockout or cGAS(G212A/S213A) cells. Upon DNA challenge, innate immune activation appeared to be mildly reduced upon expression of cGAS(P261H) compared with cGAS(WT). Finally, DNA challenge of PBMCs from donors homozygously expressing rs610913 provoked a trend toward a slightly reduced type I IFN response as compared with PBMCs from GG donors. Taken together, the steady-state activity of cGAS maintains a baseline antiviral state rendering cells more refractory to IFN-stimulated gene-sensitive viral infections. rs610913 failed to grossly differ phenotypically from the WT gene, suggesting that cGAS(P261H) and WT cGAS share a similar ability to sense viral infections in vivo.

Programmed Cell Death-Dependent Host Defense in Ocular Herpes Simplex Virus Infection

Herpes simplex virus type 1 (HSV1) remains one of the most ubiquitous human pathogens on earth. The classical presentation of HSV1 infection occurs as a recurrent lesions of the oral mucosa commonly refer to as the common cold sore. However, HSV1 also is responsible for a range of ocular diseases in immunocompetent persons that are of medical importance, causing vision loss that may result in blindness. These include a recurrent corneal disease, herpes stromal keratitis, and a retinal disease, acute retinal necrosis, for which clinically relevant animal models exist. Diverse host immune mechanisms mediate control over herpesviruses, sustaining lifelong latency in neurons. Programmed cell death (PCD) pathways including apoptosis, necroptosis, and pyroptosis serve as an innate immune mechanism that eliminates virus-infected cells and regulates infection-associated inflammation during virus invasion. These different types of cell death operate under distinct regulatory mechanisms but all server to curtail virus infection. Herpesviruses, including HSV1, have evolved numerous cell death evasion strategies that restrict the hosts ability to control PCD to subvert clearance of infection and modulate inflammation. In this review, we discuss the key studies that have contributed to our current knowledge of cell death pathways manipulated by HSV1 and relate the contributions of cell death to infection and potential ocular disease outcomes.

A Pseudorabies Virus Serine/Threonine Kinase, US3, Promotes Retrograde Transport in Axons via Akt/mToRC1

Infection of peripheral axons by alpha herpesviruses (AHVs) is a critical stage in establishing a lifelong infection in the host. Upon entering the cytoplasm of axons, AHV nucleocapsids and associated inner-tegument proteins must engage the cellular retrograde transport machinery to promote the long-distance movement of virion components to the nucleus. The current model outlining this process is incomplete, and further investigation is required to discover all viral and cellular determinants involved as well as the temporality of the events. Using a modified trichamber system, we have discovered a novel role of the pseudorabies virus (PRV) serine/threonine kinase US3 in promoting efficient retrograde transport of nucleocapsids. We discovered that transporting nucleocapsids move at similar velocities in both the presence and absence of a functional US3 kinase; however, fewer nucleocapsids are moving when US3 is absent, and they move for shorter periods of time before stopping, suggesting that US3 is required for efficient nucleocapsid engagement with the retrograde transport machinery. This led to fewer nucleocapsids reaching the cell bodies to produce a productive infection 12 h later. Furthermore, US3 was responsible for the induction of local translation in axons as early as 1 h postinfection (hpi) through the stimulation of a phosphatidylinositol 3-kinase (PI3K)/Akt-mToRC1 pathway. These data describe a novel role for US3 in the induction of local translation in axons during AHV infection, a critical step in transport of nucleocapsids to the cell body. IMPORTANCE Neurons are highly polarized cells with axons that can reach centimeters in length. Communication between axons at the periphery and the distant cell body is a relatively slow process involving the active transport of chemical messengers. There is a need for axons to respond rapidly to extracellular stimuli. Translation of repressed mRNAs present within the axon occurs to enable rapid, localized responses independently of the cell body. AHVs have evolved a way to hijack local translation in the axons to promote their transport to the nucleus. We have determined the cellular mechanism and viral components involved in the induction of axonal translation. The US3 serine/threonine kinase of PRV activates Akt-mToRC1 signaling pathways early during infection to promote axonal translation. When US3 is not present, the number of moving nucleocapsids and their processivity are reduced, suggesting that US3 activity is required for efficient engagement of nucleocapsids with the retrograde transport machinery.

Infection-induced chromatin modifications facilitate translocation of herpes simplex virus capsids to the inner nuclear membrane

Herpes simplex virus capsids are assembled and packaged in the nucleus and move by diffusion through the nucleoplasm to the nuclear envelope for egress. Analyzing their motion provides conclusions not only on capsid transport but also on the properties of the nuclear environment during infection. We utilized live-cell imaging and single-particle tracking to characterize capsid motion relative to the host chromatin. The data indicate that as the chromatin was marginalized toward the nuclear envelope it presented a restrictive barrier to the capsids. However, later in infection this barrier became more permissive and the probability of capsids to enter the chromatin increased. Thus, although chromatin marginalization initially restricted capsid transport to the nuclear envelope, a structural reorganization of the chromatin counteracted that to promote capsid transport later. Analyses of capsid motion revealed that it was subdiffusive, and that the diffusion coefficients were lower in the chromatin than in regions lacking chromatin. In addition, the diffusion coefficient in both regions increased during infection. Throughout the infection, the capsids were never enriched at the nuclear envelope, which suggests that instead of nuclear export the transport through the chromatin is the rate-limiting step for the nuclear egress of capsids. This provides motivation for further studies by validating the importance of intranuclear transport to the life cycle of HSV-1.

Herpesviruses assimilate kinesin to produce motorized viral particles

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

Pathogenesis and virulence of herpes simplex virus

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

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

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

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

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

Nano-Particles Carried by Multiple Dynein Motors Self-Regulate Their Number of Actively Participating Motors

Intra-cellular active transport by native cargos is ubiquitous. We investigate the motion of spherical nano-particles (NPs) grafted with flexible polymers that end with a nuclear localization signal peptide. This peptide allows the recruitment of several mammalian dynein motors from cytoplasmic extracts. To determine how motor–motor interactions influenced motility on the single microtubule level, we conducted bead-motility assays incorporating surface adsorbed microtubules and combined them with model simulations that were based on the properties of a single dynein. The experimental and simulation results revealed long time trajectories: when the number of NP-ligated motors N_m increased, run-times and run-lengths were enhanced and mean velocities were somewhat decreased. Moreover, the dependence of the velocity on run-time followed a universal curve, regardless of the system composition. Model simulations also demonstrated left- and right-handed helical motion and revealed self-regulation of the number of microtubule-bound, actively transporting dynein motors. This number was stochastic along trajectories and was distributed mainly between one, two, and three motors, regardless of N_m. We propose that this self-regulation allows our synthetic NPs to achieve persistent motion that is associated with major helicity. Such a helical motion might affect obstacle bypassing, which can influence active transport efficiency when facing the crowded environment of the cell.

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.

The Viral Capsid: A Master Key to Access the Host Nucleus

Viruses are pathogens that have evolved to hijack the cellular machinery to replicate themselves and spread to new cells. During the course of evolution, viruses developed different strategies to overcome the cellular defenses and create new progeny. Among them, some RNA and many DNA viruses require access to the nucleus to replicate their genome. In non-dividing cells, viruses can only access the nucleus through the nuclear pore complex (NPC). Therefore, viruses have developed strategies to usurp the nuclear transport machinery and gain access to the nucleus. The majority of these viruses use the capsid to manipulate the nuclear import machinery. However, the particular tactics employed by each virus to reach the host chromatin compartment are very different. Nevertheless, they all require some degree of capsid remodeling. Recent notions on the interplay between the viral capsid and cellular factors shine new light on the quest for the nuclear entry step and for the fate of these viruses. In this review, we describe the main components and function of nuclear transport machinery. Next, we discuss selected examples of RNA and DNA viruses (HBV, HSV, adenovirus, and HIV) that remodel their capsid as part of their strategies to access the nucleus and to replicate.

Computational modeling of dynein motor proteins at work

Along with various experimental methods, a combination of theoretical and computational methods is essential to explore different length-scale and time-scale processes in the biological system. The functional mechanism of a dynein, an ATP-fueled motor protein, working in a multiprotein complex, involves a wide range of length/time-scale events. It generates mechanical force from chemical energy and moves on microtubules towards the minus end direction while performing a large number of biological processes including ciliary beating, intracellular material transport, and cell division. Like in the cases of other conventional motor proteins, a combination of experimental techniques including X-crystallography, cryo-electron microscopy, and single molecular assay have provided a wealth of information about the mechanochemical cycle of a dynein. Dyneins have a large and complex structural architecture and therefore, computational modeling of different aspects of a dynein is extremely challenging. As the process of dynein movement involves varying length and timescales, it demands, like in experiments, a combination of computational methods covering such a wide range of processes for the comprehensive investigation of the mechanochemical cycle. In this review article, we will summarize how the use of state-of-the-art computational methods can provide a detailed molecular understanding of the mechanochemical cycle of the dynein. We implemented all-atom molecular dynamics simulations and hybrid quantum-mechanics/molecular-mechanics simulations to explore the ATP hydrolysis mechanisms at the primary ATPase site (AAA1) of dynein. To investigate the large-scale conformational changes we employed coarse-grained structure-based molecular dynamics simulations to capture the domain motions. Here we explored the conformational changes upon binding of ATP at AAA1, nucleotide state-dependent regulation of the mechanochemical cycle, and inter-head coordination by inter-head tension. Additionally, implementing a phenomenological theoretical model we explore the force-dependent detachment rate of a motorhead from the microtubule and the principle of multi-dynein cooperation during cargo transport.

A Ran-binding protein facilitates nuclear import of human papillomavirus type 16

Human papillomaviruses (HPVs) utilize an atypical mode of nuclear import during cell entry. Residing in the Golgi apparatus until mitosis onset, a subviral complex composed of the minor capsid protein L2 and viral DNA (L2/vDNA) is imported into the nucleus after nuclear envelope breakdown by associating with mitotic chromatin. In this complex, L2 plays a crucial role in the interactions with cellular factors that enable delivery and ultimately tethering of the viral genome to mitotic chromatin. To date, the cellular proteins facilitating these steps remain unknown. Here, we addressed which cellular proteins may be required for this process. Using label-free mass spectrometry, biochemical assays, microscopy, and functional virological assays, we discovered that L2 engages a hitherto unknown protein complex of Ran-binding protein 10 (RanBP10), karyopherin alpha2 (KPNA2), and dynein light chain DYNLT3 to facilitate transport towards mitotic chromatin. Thus, our study not only identifies novel cellular interactors and mechanism that facilitate a poorly understood step in HPV entry, but also a novel cellular transport complex.

Human papillomaviruses (HPVs) cause proliferative lesions such as benign warts or malignant invasive cancers. Like other DNA viruses, HPV has to deliver its genome to the nucleus for viral genome transcription and replication. After initial attachment, HPVs are endocytosed to be eventually directed to the trans-Golgi-network (TGN) by intracellular trafficking, where they reside until cell division. Mitosis onset enables access of the virus to cellular chromatin after nuclear envelope breakdown. Tethering of the virus to mitotic chromatin ensures nuclear delivery upon reformation of the nuclear envelope after mitosis. Our previous work showed that the minor capsid protein L2 facilitates nuclear delivery. However, the detailed mechanism, namely, how HPV trafficks from cytosol to the nuclear space, is barely understood. Here, we identified for the first time cellular proteins that interacted with L2 for nuclear import. Mechanistically, the proteins formed a hitherto unknown cellular transport complex that interacted with L2 to direct the virus to mitotic chromosomes by microtubular transport. Our findings provided not only evidence for a transport mechanism of a poorly understood step of HPV entry, but also discovered a novel cellular transport complex.

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

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

Dewormer drug fenbendazole has antiviral effects on BoHV-1 productive infection in cell cultures

BACKGROUND

Fenbendazole, a dewormer drug, is used widely in the clinical treatment of parasite infections in animals. Recent studies have shown that fenbendazole has substantial effects on tumor growth, immune responses, and inflammatory responses, suggesting that fenbendazole is a pluripotent drug. Nevertheless, the antiviral effects have not been reported. Fenbendazole can disrupt microtubules, which are essential for multiple viruses infections, suggesting that fenbendazole might have antiviral effects.

OBJECTIVES

This study examined whether fenbendazole could inhibit bovine herpesvirus 1 (BoHV-1) productive infection in cell cultures.

METHODS

The effects of fenbendazole on viral production, transcription of the immediate early (IE) genes, viron-associated protein expression, and the cellular signaling PLC-γ1/Akt pathway were assessed using distinct methods.

RESULTS

Fenbendazole could inhibit BoHV-1 productive infections significantly in MDBK cells in a dose-dependent manner. A time-of-addition assay indicated that fenbendazole affected both the early and late stages in the virus replication cycles. The transcription of IE genes, including BoHV-1 infected cell protein 0 (bICP0), bCP4, and bICP22, as well as the synthesis of viron-associated proteins, were disrupted differentially by the fenbendazole treatment. The treatment did not affect the cellular signaling pathway of PLC-γ1/Akt, a known cascade playing important roles in virus infection.

CONCLUSIONS

Overall, fenbendazole has antiviral effects on BoHV-1 replication.

Viral entry and the ubiquitin-proteasome system

Viruses confiscate cellular components of the ubiquitin-proteasome system (UPS) to facilitate many aspects of the infectious cycle. The 26S proteasome is an ATP-dependent, multisubunit proteolytic machine present in all eukaryotic cells. The proteasome executes the controlled degradation of functional proteins, as well as the hydrolysis of aberrantly folded polypeptides. There is growing evidence for the role of the UPS in viral entry. The UPS assists in several steps of the initiation of infection, including endosomal escape of the entering virion, intracellular transport of incoming nucleocapsids and uncoating of the viral genome. Inhibitors of proteasome activity, including MG132, epoxomicin, lactacystin and bortezomib have been integral to developments in this area. Here, we review the mechanistic details of UPS involvement in the entry process of viruses from a multitude of families. The possibility of proteasome inhibitors as therapeutic antiviral agents is highlighted.

Absence of cGAS-mediated type I IFN responses in HIV-1-infected T cells

The DNA sensor cGAS catalyzes the production of the cyclic dinucleotide cGAMP, resulting in type I interferon responses. We addressed the functionality of cGAS-mediated DNA sensing in human and murine T cells. Activated primary CD4+ T cells expressed cGAS and responded to plasmid DNA by upregulation of ISGs and release of bioactive interferon. In mouse T cells, cGAS KO ablated sensing of plasmid DNA, and TREX1 KO enabled cells to sense short immunostimulatory DNA. Expression of IFIT1 and MX2 was downregulated and upregulated in cGAS KO and TREX1 KO T cell lines, respectively, compared to parental cells. Despite their intact cGAS sensing pathway, human CD4+ T cells failed to mount a reverse transcriptase (RT) inhibitor-sensitive immune response following HIV-1 infection. In contrast, infection of human T cells with HSV-1 that is functionally deficient for the cGAS antagonist pUL41 (HSV-1ΔUL41N) resulted in a cGAS-dependent type I interferon response. In accordance with our results in primary CD4+ T cells, plasmid challenge or HSV-1ΔUL41N inoculation of T cell lines provoked an entirely cGAS-dependent type I interferon response, including IRF3 phosphorylation and expression of ISGs. In contrast, no RT-dependent interferon response was detected following transduction of T cell lines with VSV-G-pseudotyped lentiviral or gammaretroviral particles. Together, T cells are capable to raise a cGAS-dependent cell-intrinsic response to both plasmid DNA challenge or inoculation with HSV-1ΔUL41N. However, HIV-1 infection does not appear to trigger cGAS-mediated sensing of viral DNA in T cells, possibly by revealing viral DNA of insufficient quantity, length, and/or accessibility to cGAS.

Proteomic analysis of CHIKV-infected human fibroblast-like synoviocytes: Identification of host factors potentially associated with CHIKV replication and cellular pathogenesis

Chikungunya virus (CHIKV) is a mosquito-borne virus that causes arthralgic fever. Fibroblast-like synoviocytes play a key role in joint damage in inflammatory arthritides and can additionally serve as target cells for CHIKV infection. To gain a better understanding of CHIKV-induced arthralgia, the interaction between CHIKV and synoviocytes was investigated at the protein level. A gel-enhanced liquid chromatography-mass spectrometry (GeLC-MS/MS) approach was used to examine protein expression from primary human fibroblast-like synoviocytes (HFLS) infected with clinical isolates of CHIKV at 12 and 24 hr post infection. Our analysis identified 259 and 241 proteins of known function that were differentially expressed (>1.5 or <-1.5 fold change) following CHIKV infection at 12 and 24 hpi, respectively. These proteins are involved in cellular homeostasis, including cellular trafficking, cytoskeletal organization, immune response, metabolic process, and protein modification. Some of these proteins have previously been reported to participate in arthralgia/arthritis and the death of infected cells. Our results provide information on the CHIKV-induced modulation of cellular proteins of HFLS at an early stage of infection, as well as highlighting biological processes associated with CHIKV infection in the main target cells of the joint.

FAK regulates dynein localisation and cell polarity in migrating mouse fibroblasts

BACKGROUND

Fibroblasts executing directional migration position their centrosome, and their Golgi apparatus, in front of the nucleus towards the cell leading edge. Centrosome positioning relative to the nucleus has been associated to mechanical forces exerted on the centrosome by the microtubule-dependent molecular motor cytoplasmic dynein 1, and to nuclear movements such as rearward displacement and rotation events. Dynein has been proposed to regulate the position of the centrosome by exerting pulling forces on microtubules from the cell leading edge, where the motor is enriched during migration. However, the mechanism explaining how dynein acts at the front of the cells has not been elucidated.

RESULTS

We present here results showing that the protein Focal Adhesion Kinase (FAK) interacts with dynein and regulates the enrichment of the dynein/dynactin complex at focal adhesions at the cell the leading edge of migrating fibroblasts. This suggests that focal adhesions provide anchoring sites for dynein during the polarisation process. In support of this, we present evidence indicating that the interaction between FAK and dynein, which is regulated by the phosphorylation of FAK on its Ser732 residue, is required for proper centrosome positioning. Our results further show that the polarisation of the centrosome can occur independently of nuclear movements. Although FAK regulates both nuclear and centrosome motilities, downregulating the interaction between FAK and dynein affects only the nuclear independent polarisation of the centrosome.

CONCLUSIONS

Our work highlights the role of FAK as a key player in the regulation of several aspects of cell polarity. We thus propose a model in which the transient localisation of dynein with focal adhesions provides a tuneable mechanism to bias dynein traction forces on microtubules allowing proper centrosome positioning in front of the nucleus.

SIGNIFICANCE

We unravel here a new role for the cancer therapeutic target FAK in the regulation of cell morphogenesis.

Role of AAA3 Domain in Allosteric Communication of Dynein Motor Proteins

Cytoplasmic dynein, an AAA+ motif containing motor, generates force and movement along the microtubule to execute important biological functions including intracellular material transport and cell division by hydrolyzing ATP. Among the six AAA+ domains, AAA1 is the primary ATPase site where a single ATP hydrolysis generates a single step. Nucleotide states in AAA3 gate dynein's activity, suggesting that AAA3 acts as a regulatory switch. However, the comprehensive structural perspective of AAA3 in dynein's mechanochemical cycle remains unclear. Here, we explored the allosteric transition path of dynein involving AAA3 using a coarse-grained structure-based model. ATP binding to the AAA1 domain creates a cascade of conformational changes through the other domains of the ring, which leads to the pre-power stroke formation. The linker domain, which is the mechanical element of dynein, shifts from a straight to a bent conformation during this process. In our present study, we observe that AAA3 gates the allosteric communication from AAA1 to the microtubule binding domain (MTBD) through AAA4 and AAA5. The MTBD is linked to the AAA+ ring via a coiled-coil stalk and a buttress domain, which are extended from AAA4 and AAA5, respectively. Further analysis also uncovers the role of AAA3 in the linker movement. The free energy calculation shows that the linker prefers the straight conformation when AAA3 remains in the ATP-bound condition. As AAA3 restricts the motion of AAA4 and AAA5, the linker/AAA5 interactions get stabilized, and the linker cannot move to the pre-power stroke state that halts the complete structural transition required for the mechanochemical cycle. Therefore, we suggest that AAA3 governs dynein's mechanochemical cycle and motility by controlling the AAA4 and AAA5 domains that further regulate the linker movement and the power stroke formation.

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.

Herpes Simplex Virus Type 1 Propagation, Titration and Single-step Growth Curves

Given the endemic seroprevalence of herpes simplex viruses (HSV), its associated human diseases, and the emergence of acyclovir-resistant strains, there is a continuous need for better antiviral therapies. Towards this aim, identifying mechanistic details of how HSV-1 manipulates infected cells, how it modulates the immune responses, and how it causes diseases are essential. Measuring titers and growth kinetics of clinical isolates and viral mutants are important for a thorough characterization of viral phenotypes in vitro and in vivo. We provide protocols for the preparation as well as titration of HSV-1 stocks, and explain how to perform single-step growth curves to characterize the functions of viral proteins or host factors during infection. In particular, we describe methods to prepare and characterize high-titer HSV-1 stocks with low genome to titer ratios that are required for infection studies in cell culture and animal experiments.

HSV-1 triggers paracrine fibroblast growth factor response from cortical brain cells via immediate-early protein ICP0

BACKGROUND

Herpes simplex virus-1 (HSV-1) infections of the central nervous system (CNS) can result in HSV-1 encephalitis (HSE) which is characterized by severe brain damage and long-term disabilities. Different cell types including neurons and astrocytes become infected in the course of an HSE which leads to an activation of glial cells. Activated glial cells change their neurotrophic factor profile and modulate inflammation and repair. The superfamily of fibroblast growth factors (FGFs) is one of the largest family of neurotrophic factors comprising 22 ligands. FGFs induce pro-survival signaling in neurons and an anti-inflammatory answer in glial cells thereby providing a coordinated tissue response which favors repair over inflammation. Here, we hypothesize that FGF expression is altered in HSV-1-infected CNS cells.

METHOD

We employed primary murine cortical cultures comprising a mixed cell population of astrocytes, neurons, microglia, and oligodendrocytes. Astrocyte reactivity was morphometrically monitored by an automated image analysis algorithm as well as by analyses of A1/A2 marker expression. Altered FGF expression was detected by quantitative real-time PCR and its paracrine FGF activity. In addition, HSV-1 mutants were employed to characterize viral factors important for FGF responses of infected host cells.

RESULTS

Astrocytes in HSV-1-infected cortical cultures were transiently activated and became hypertrophic and expressed both A1- and A2-markers. Consistently, a number of FGFs were transiently upregulated inducing paracrine neurotrophic signaling in neighboring cells. Most prominently, FGF-4, FGF-8, FGF-9, and FGF-15 became upregulated in a switch-on like mechanism. This effect was specific for CNS cells and for a fully functional HSV-1. Moreover, the viral protein ICP0 critically mediated the FGF switch-on mechanism.

CONCLUSIONS

HSV-1 uses the viral protein ICP0 for the induction of FGF-expression in CNS cells. Thus, we propose that HSV-1 triggers FGF activity in the CNS for a modulation of tissue response upon infection.

Viral, bacterial, and fungal infections of the oral mucosa: Types, incidence, predisposing factors, diagnostic algorithms, and management

For millions of years, microbiota residing within us, including those in the oral cavity, coexisted in a harmonious symbiotic fashion that provided a quintessential foundation for human health. It is now clear that disruption of such a healthy relationship leading to microbial dysbiosis causes a wide array of infections, ranging from localized, mild, superficial infections to deep, disseminated life-threatening diseases. With recent advances in research, diagnostics, and improved surveillance we are witnessing an array of emerging and re-emerging oral infections and orofacial manifestations of systemic infections. Orofacial infections may cause significant discomfort to the patients and unnecessary economic burden. Thus, the early recognition of such infections is paramount for holistic patient management, and oral clinicians have a critical role in recognizing, diagnosing, managing, and preventing either new or old orofacial infections. This paper aims to provide an update on current understanding of well-established and emerging viral, bacterial, and fungal infections manifesting in the human oral cavity.

Herpes Simplex Virus, Alzheimer's Disease and a Possible Role for Rab GTPases

Herpes simplex virus (HSV) is a common pathogen, infecting 85% of adults in the United States. After reaching the nucleus of the long-lived neuron, HSV may enter latency to persist throughout the life span. Re-activation of latent herpesviruses is associated with progressive cognitive impairment and Alzheimer's disease (AD). As an enveloped DNA virus, HSV exploits cellular membrane systems for its life cycle, and thereby comes in contact with the Rab family of GTPases, master regulators of intracellular membrane dynamics. Knock-down and overexpression of specific Rabs reduce HSV production. Disheveled membrane compartments could lead to AD because membrane sorting and trafficking are crucial for synaptic vesicle formation, neuronal survival signaling and Abeta production. Amyloid precursor protein (APP), a transmembrane glycoprotein, is the parent of Abeta, the major component of senile plaques in AD. Up-regulation of APP expression due to HSV is significant since excess APP interferes with Rab5 endocytic trafficking in neurons. Here, we show that purified PC12-cell endosomes transport both anterograde and retrograde when injected into the squid giant axon at rates similar to isolated HSV. Intracellular HSV co-fractionates with these endosomes, contains APP, Rab5 and TrkA, and displays a second membrane. HSV infected PC12 cells up-regulate APP expression. Whether interference with Rabs has a specific effect on HSV or indirectly affects membrane compartment dynamics co-opted by virus needs further study. Ultimately Rabs, their effectors or their membrane-binding partners may serve as handles to reduce the impact of viral re-activation on cognitive function, or even as more general-purpose anti-microbial therapies.

Setting the dynein motor in motion: New insights from electron tomography

Dyneins are ATP-fueled macromolecular machines that power all minus-end microtubule-based transport processes of molecular cargo within eukaryotic cells and play essential roles in a wide variety of cellular functions. These complex and fascinating motors have been the target of countless structural and biophysical studies. These investigations have elucidated the mechanism of ATP-driven force production and have helped unravel the conformational rearrangements associated with the dynein mechanochemical cycle. However, despite decades of research, it remains unknown how these molecular motions are harnessed to power massive cellular reorganization and what are the regulatory mechanisms that drive these processes. Recent advancements in electron tomography imaging have enabled researchers to visualize dynein motors in their transport environment with unprecedented detail and have led to exciting discoveries regarding dynein motor function and regulation. In this review, we will highlight how these recent structural studies have fundamentally propelled our understanding of the dynein motor and have revealed some unexpected, unifying mechanisms of regulation.

Viral Capsid Trafficking along Treadmilling Tubulin Filaments in Bacteria

Cargo trafficking along microtubules is exploited by eukaryotic viruses, but no such examples have been reported in bacteria. Several large Pseudomonas phages assemble a dynamic, tubulin-based (PhuZ) spindle that centers replicating phage DNA sequestered within a nucleus-like structure. Here, we show that capsids assemble on the membrane and then move rapidly along PhuZ filaments toward the phage nucleus for DNA packaging. The spindle rotates the phage nucleus, distributing capsids around its surface. PhuZ filaments treadmill toward the nucleus at a constant rate similar to the rate of capsid movement and the linear velocity of nucleus rotation. Capsids become trapped along mutant static PhuZ filaments that are defective in GTP hydrolysis. Our results suggest a transport and distribution mechanism in which capsids attached to the sides of filaments are trafficked to the nucleus by PhuZ polymerization at the poles, demonstrating that the phage cytoskeleton evolved cargo-trafficking capabilities in bacteria.

Herpesvirus acts with the cytoskeleton and promotes cancer progression

The cytoskeleton is a complex fibrous reticular structure composed of microfilaments, microtubules and intermediate filaments. These components coordinate morphology support and intracellular transport that is involved in a variety of cell activities, such as cell proliferation, migration and differentiation. In addition, the cytoskeleton also plays an important role in viral infection. During an infection by a Herpesvirus, the virus utilizes microfilaments to enter cells and travel to the nucleus by microtubules; the viral DNA replicates with the help of host microfilaments; and the virus particles start assembling with a capsid in the cytoplasm before egress. The cytoskeleton changes in cells infected with Herpesvirus are made to either counteract or obey the virus, thereby promote cell transforming into cancerous ones. This article aims to clarify the interaction between the virus and cytoskeleton components in the process of Herpesvirus infection and the molecular motor, cytoskeleton-associated proteins and drugs that play an important role in the process of a Herpesvirus infection and carcinogenesis process.

Host proteins involved in microtubule-dependent HIV-1 intracellular transport and uncoating

Microtubules and microtubule-associated proteins are critical for cargo transport throughout the cell. Many viruses are able to usurp these transport systems for their own replication and spread. HIV-1 utilizes these proteins for many of its early events postentry, including transport, uncoating and reverse transcription. The molecular motor proteins dynein and kinesin-1 are the primary drivers of cargo transport, and HIV-1 utilizes these proteins for infection. In this Review, we highlight recent developments in the understanding of how HIV-1 hijacks motor transport, the key cellular and viral proteins involved, and the ways that transport influences other steps in the HIV-1 lifecycle.

Herpes Simplex Virus Evasion of Early Host Antiviral Responses

Herpes simplex viruses type 1 (HSV-1) and type 2 (HSV-2) have co-evolved with humans for thousands of years and are present at a high prevalence in the population worldwide. HSV infections are responsible for several illnesses including skin and mucosal lesions, blindness and even life-threatening encephalitis in both, immunocompetent and immunocompromised individuals of all ages. Therefore, diseases caused by HSVs represent significant public health burdens. Similar to other herpesviruses, HSV-1 and HSV-2 produce lifelong infections in the host by establishing latency in neurons and sporadically reactivating from these cells, eliciting recurrences that are accompanied by viral shedding in both, symptomatic and asymptomatic individuals. The ability of HSVs to persist and recur in otherwise healthy individuals is likely given by the numerous virulence factors that these viruses have evolved to evade host antiviral responses. Here, we review and discuss molecular mechanisms used by HSVs to evade early innate antiviral responses, which are the first lines of defense against these viruses. A comprehensive understanding of how HSVs evade host early antiviral responses could contribute to the development of novel therapies and vaccines to counteract these viruses.

Equine Herpesvirus 1 Bridles T Lymphocytes To Reach Its Target Organs

Equine herpesvirus 1 (EHV1) is an ancestral alphaherpesvirus that is related to herpes simplex virus 1 and causes respiratory, reproductive, and neurological disorders in Equidae. EHV1 is indisputably a master at exploiting leukocytes to reach its target organs, accordingly evading the host immunity. However, the role of T lymphocytes in cell-associated viremia remains poorly understood. Here we show that activated T lymphocytes efficiently become infected and support viral replication despite the presence of protective immunity. We demonstrate a restricted expression of viral proteins on the surfaces of infected T cells, which prevents immune recognition. In addition, we indicate a hampered release of progeny, which results in the accumulation of nucleocapsids in the T cell nucleus. Upon engagement with the target endothelium, late viral proteins orchestrate viral synapse formation and viral transfer to the contact cell. Our findings have significant implications for the understanding of EHV1 pathogenesis, which is essential for developing innovative therapies to prevent the devastating clinical symptoms of infection.

Equine herpesvirus 1 (EHV1) replicates in the respiratory epithelium and disseminates through the body via a cell-associated viremia in leukocytes, despite the presence of neutralizing antibodies. “Hijacked” leukocytes, previously identified as monocytic cells and T lymphocytes, transmit EHV1 to endothelial cells of the endometrium or central nervous system, causing reproductive (abortigenic variants) or neurological (neurological variants) disorders. In the present study, we questioned the potential route of EHV1 infection of T lymphocytes and how EHV1 misuses T lymphocytes as a vehicle to reach the endothelium of the target organs in the absence or presence of immune surveillance. Viral replication was evaluated in activated and quiescent primary T lymphocytes, and the results demonstrated increased infection of activated versus quiescent, CD4⁺ versus CD8⁺, and blood- versus lymph node-derived T cells. Moreover, primarily infected respiratory epithelial cells and circulating monocytic cells efficiently transferred virions to T lymphocytes in the presence of neutralizing antibodies. Albeit T-lymphocytes express all classes of viral proteins early in infection, the expression of viral glycoproteins on their cell surface was restricted. In addition, the release of viral progeny was hampered, resulting in the accumulation of viral nucleocapsids in the T cell nucleus. During contact of infected T lymphocytes with endothelial cells, a late viral protein(s) orchestrates T cell polarization and synapse formation, followed by anterograde dynein-mediated transport and transfer of viral progeny to the engaged cell. This represents a sophisticated but efficient immune evasion strategy to allow transfer of progeny virus from T lymphocytes to adjacent target cells. These results demonstrate that T lymphocytes are susceptible to EHV1 infection and that cell-cell contact transmits infectious virus to and from T lymphocytes.

IMPORTANCE Equine herpesvirus 1 (EHV1) is an ancestral alphaherpesvirus that is related to herpes simplex virus 1 and causes respiratory, reproductive, and neurological disorders in Equidae. EHV1 is indisputably a master at exploiting leukocytes to reach its target organs, accordingly evading the host immunity. However, the role of T lymphocytes in cell-associated viremia remains poorly understood. Here we show that activated T lymphocytes efficiently become infected and support viral replication despite the presence of protective immunity. We demonstrate a restricted expression of viral proteins on the surfaces of infected T cells, which prevents immune recognition. In addition, we indicate a hampered release of progeny, which results in the accumulation of nucleocapsids in the T cell nucleus. Upon engagement with the target endothelium, late viral proteins orchestrate viral synapse formation and viral transfer to the contact cell. Our findings have significant implications for the understanding of EHV1 pathogenesis, which is essential for developing innovative therapies to prevent the devastating clinical symptoms of infection.

Characterization of the Filovirus-Resistant Cell Line SH-SY5Y Reveals Redundant Role of Cell Surface Entry Factors

Filoviruses infect a wide range of cell types with the exception of lymphocytes. The intracellular proteins cathepsin B and L, two-pore channel 1 and 2, and bona fide receptor Niemann–Pick Disease C1 (NPC1) are essential for the endosomal phase of cell entry. However, earlier steps of filoviral infection remain poorly characterized. Numerous plasma membrane proteins have been implicated in attachment but it is still unclear which ones are sufficient for productive entry. To define a minimal set of host factors required for filoviral glycoprotein-driven cell entry, we screened twelve cell lines and identified the nonlymphocytic cell line SH-SY5Y to be specifically resistant to filovirus infection. Heterokaryons of SH-SY5Y cells fused to susceptible cells were susceptible to filoviruses, indicating that SH-SY5Y cells do not express a restriction factor but lack an enabling factor critical for filovirus entry. However, all tested cell lines expressed functional intracellular factors. Global gene expression profiling of known cell surface entry factors and protein expression levels of analyzed attachment factors did not reveal any correlation between susceptibility and expression of a specific host factor. Using binding assays with recombinant filovirus glycoprotein, we identified cell attachment as the step impaired in filovirus entry in SH-SY5Y cells. Individual overexpression of attachment factors T-cell immunoglobulin and mucin domain 1 (TIM-1), Axl, Mer, or dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN) rendered SH-SY5Y cells susceptible to filovirus glycoprotein-driven transduction. Our study reveals that a lack of attachment factors limits filovirus entry and provides direct experimental support for a model of filoviral cell attachment where host factor usage at the cell surface is highly promiscuous.

Autophagic degradation of lamins facilitates the nuclear egress of herpes simplex virus type 1

Turan and Grosche et al. show that herpes simplex virus type 1 (HSV-1) infection leads to autophagic degradation of nuclear lamins in immature dendritic cells, facilitating HSV-1 nuclear egress and the formation of infectious progeny virus. In mature dendritic cells, autophagy is inhibited due to elevated KIF1B and KIF2A protein levels.

Dendritic cells (DCs) are crucial for the induction of potent antiviral immune responses. In contrast to immature DCs (iDCs), mature DCs (mDCs) are not permissive for infection with herpes simplex virus type 1 (HSV-1). Here, we demonstrate that HSV-1 infection of iDCs and mDCs induces autophagy, which promotes the degradation of lamin A/C, B1, and B2 in iDCs only. This in turn facilitates the nuclear egress of progeny viral capsids and thus the formation of new infectious particles. In contrast, lamin protein levels remain stable in HSV-1–infected mDCs due to an inefficient autophagic flux. Elevated protein levels of KIF1B and KIF2A in mDCs inhibited lamin degradation, likely by hampering autophagosome–lysosome fusion. Therefore, in mDCs, fewer progeny capsids were released from the nuclei into the cytosol, and fewer infectious virions were assembled. We hypothesize that inhibition of autophagic lamin degradation in mDCs represents a very powerful cellular counterstrike to inhibit the production of progeny virus and thus viral spread.

Experimental Dissection of the Lytic Replication Cycles of Herpes Simplex Viruses in vitro

Herpes simplex viruses type 1 and type 2 (HSV-1 and HSV-2) produce lifelong infections and are highly prevalent in the human population. Both viruses elicit numerous clinical manifestations and produce mild-to-severe diseases that affect the skin, eyes, and brain, among others. Despite the existence of numerous antivirals against HSV, such as acyclovir and acyclovir-related analogs, virus variants that are resistant to these compounds can be isolated from immunosuppressed individuals. For such isolates, second-line drugs can be used, yet they frequently produce adverse side effects. Furthermore, topical antivirals for treating cutaneous HSV infections usually display poor to moderate efficacy. Hence, better or novel anti-HSV antivirals are needed and details on their mechanisms of action would be insightful for improving their efficacy and identifying specific molecular targets. Here, we review and dissect the lytic replication cycles of herpes simplex viruses, discussing key steps involved in cell infection and the processes that yield new virions. Additionally, we review and discuss rapid, easy-to-perform and simple experimental approaches for studying key steps involved in HSV replication to facilitate the identification of the mechanisms of action of anti-HSV compounds.

Interaction between the cellular E3 ubiquitin ligase SIAH-1 and the viral immediate-early protein ICP0 enables efficient replication of Herpes Simplex Virus type 2 in vivo

Herpes Simplex Virus type 2 (HSV-2) is a neurotropic human pathogen. Upon de novo infection, the viral infected cell protein 0 (ICP0) is immediately expressed and interacts with various cellular components during the viral replication cycle. ICP0 is a multifunctional regulatory protein that has been shown to be important for both efficient viral replication and virus reactivation from latency. In particular, as previously demonstrated in transfected tissue culture models, ICP0 interacts with the cellular E3 ubiquitin ligase SIAH-1, which targets ICP0 for proteasomal degradation. However, the consequence of this virus-host interaction during the establishment of HSV-2 infection in vivo has not yet been elucidated. Here we confirmed that ICP0 of HSV-2 interacts with SIAH-1 via two conserved PxAxVxP amino acid binding motifs. We also demonstrate in vitro that a SIAH-1 binding-deficient HSV-2 strain, constructed by homologous recombination technology, exhibits an attenuated growth curve and impaired DNA and protein synthesis. This attenuated phenotype was also confirmed in an in vivo ocular infection mouse model. Specifically, viral load of the SIAH-1 binding-deficient HSV-2 mutant was significantly reduced in the trigeminal ganglia and brain stem at day 5 and 7 post infection. Our findings indicate that the interplay between ICP0 and SIAH-1 is important for efficient HSV-2 replication in vivo, thereby affecting viral dissemination kinetics in newly infected organisms, and possibly revealing novel targets for antiviral therapy.

Phototracking Vaccinia Virus Transport Reveals Dynamics of Cytoplasmic Dispersal and a Requirement for A36R and F12L for Exit from the Site of Wrapping

The microtubule cytoskeleton is a primary organizer of viral infections for delivering virus particles to their sites of replication, establishing and maintaining subcellular compartments where distinct steps of viral morphogenesis take place, and ultimately dispersing viral progeny. One of the best characterized examples of virus motility is the anterograde transport of the wrapped virus form of vaccinia virus (VACV) from the trans-Golgi network (TGN) to the cell periphery by kinesin-1. Yet many aspects of this transport event are elusive due to the speed of motility and the challenges of imaging this stage at high resolution over extended time periods. We have established a novel imaging technology to track virus transport that uses photoconvertible fluorescent recombinant viruses to track subsets of virus particles from their site of origin and determine their destination. Here we image virus exit from the TGN and their rate of egress to the cell periphery. We demonstrate a role for kinesin-1 engagement in regulating virus exit from the TGN by removing A36 and F12 function, critical viral mediators of kinesin-1 recruitment to virus particles. Phototracking viral particles and components during infection is a powerful new imaging approach to elucidate mechanisms of virus replication.