Pressure-driven release of viral genome into a host nucleus is a mechanism leading to herpes infection

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.

The battle between host antiviral innate immunity and immune evasion by cytomegalovirus

Cytomegalovirus (CMV) has successfully established a long-lasting latent infection in humans due to its ability to counteract the host antiviral innate immune response. During coevolution with the host, the virus has evolved various evasion techniques to evade the host's innate immune surveillance. At present, there is still no vaccine available for the prevention and treatment of CMV infection, and the interaction between CMV infection and host antiviral innate immunity is still not well understood. However, ongoing studies will offer new insights into how to treat and prevent CMV infection and its related diseases. Here, we update recent studies on how CMV evades antiviral innate immunity, with a focus on how CMV proteins target and disrupt critical adaptors of antiviral innate immune signaling pathways. This review also discusses some classic intrinsic cellular defences that are crucial to the fight against viral invasion. A comprehensive review of the evasion mechanisms of antiviral innate immunity by CMV will help investigators identify new therapeutic targets and develop vaccines against CMV infection.

Models of Herpes Simplex Virus Latency

Our current understanding of HSV latency is based on a variety of clinical observations, and in vivo, ex vivo, and in vitro model systems, each with unique advantages and drawbacks. The criteria for authentically modeling HSV latency include the ability to easily manipulate host genetics and biological pathways, as well as mimicking the immune response and viral pathogenesis in human infections. Although realistically modeling HSV latency is necessary when choosing a model, the cost, time requirement, ethical constraints, and reagent availability are also equally important. Presently, there remains a pressing need for in vivo models that more closely recapitulate human HSV infection. While the current in vivo, ex vivo, and in vitro models used to study HSV latency have limitations, they provide further insights that add to our understanding of latency. In vivo models have shed light on natural infection routes and the interplay between the host immune response and the virus during latency, while in vitro models have been invaluable in elucidating molecular pathways involved in latency. Below, we review the relative advantages and disadvantages of current HSV models and highlight insights gained through each.

The Herpes Simplex Virus pUL16 and pUL21 Proteins Prevent Capsids from Docking at Nuclear Pore Complexes

After entry into cells, herpes simplex virus (HSV) nucleocapsids dock at nuclear pore complexes (NPCs) through which viral genomes are released into the nucleoplasm where viral gene expression, genome replication, and early steps in virion assembly take place. After their assembly, nucleocapsids are translocated to the cytoplasm for final virion maturation. Nascent cytoplasmic nucleocapsids are prevented from binding to NPCs and delivering their genomes to the nucleus from which they emerged, but how this is accomplished is not understood. Here we report that HSV pUL16 and pUL21 deletion mutants accumulate empty capsids at the cytoplasmic face of NPCs late in infection. Additionally, prior expression of pUL16 and pUL21 prevented incoming nucleocapsids from docking at NPCs, delivering their genomes to the nucleus and initiating viral gene expression. Both pUL16 and pUL21 localized to the nuclear envelope, placing them in an appropriate location to interfere with nucleocapsid/NPC interactions.

Flavonoids with Anti-Herpes Simplex Virus Properties: Deciphering Their Mechanisms in Disrupting the Viral Life Cycle

The herpes simplex virus (HSV) is a double-stranded DNA human virus that causes persistent infections with recurrent outbreaks. HSV exists in two forms: HSV-1, responsible for oral herpes, and HSV-2, primarily causing genital herpes. Both types can lead to significant complications, including neurological issues. Conventional treatment, involving acyclovir and its derivatives, faces challenges due to drug resistance. This underscores the imperative for continual research and development of new drugs, with a particular emphasis on exploring the potential of natural antivirals. Flavonoids have demonstrated promise in combating various viruses, including those within the herpesvirus family. This review, delving into recent studies, reveals the intricate mechanisms by which flavonoids decode their antiviral capabilities against HSV. By disrupting key stages of the viral life cycle, such as attachment to host cells, entry, DNA replication, latency, and reactivation, flavonoids emerge as formidable contenders in the ongoing battle against HSV infections.

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.

A CRISPR-based rapid DNA repositioning strategy and the early intranuclear life of HSV-1

The relative positions of viral DNA genomes to the host intranuclear environment play critical roles in determining virus fate. Recent advances in the application of chromosome conformation capture-based sequencing analysis (3 C technologies) have revealed valuable aspects of the spatiotemporal interplay of viral genomes with host chromosomes. However, to elucidate the causal relationship between the subnuclear localization of viral genomes and the pathogenic outcome of an infection, manipulative tools are needed. Rapid repositioning of viral DNAs to specific subnuclear compartments amid infection is a powerful approach to synchronize and interrogate this dynamically changing process in space and time. Herein, we report an inducible CRISPR-based two-component platform that relocates extrachromosomal DNA pieces (5 kb to 170 kb) to the nuclear periphery in minutes (CRISPR-nuPin). Based on this strategy, investigations of herpes simplex virus 1 (HSV-1), a prototypical member of the human herpesvirus family, revealed unprecedently reported insights into the early intranuclear life of the pathogen: (I) Viral genomes tethered to the nuclear periphery upon entry, compared with those freely infecting the nucleus, were wrapped around histones with increased suppressive modifications and subjected to stronger transcriptional silencing and prominent growth inhibition. (II) Relocating HSV-1 genomes at 1 hr post infection significantly promoted the transcription of viral genes, termed an 'Escaping' effect. (III) Early accumulation of ICP0 was a sufficient but not necessary condition for 'Escaping'. (IV) Subnuclear localization was only critical during early infection. Importantly, the CRISPR-nuPin tactic, in principle, is applicable to many other DNA viruses.

Role of DNA-DNA sliding friction and nonequilibrium dynamics in viral genome ejection and packaging

Many viruses eject their DNA via a nanochannel in the viral shell, driven by internal forces arising from the high-density genome packing. The speed of DNA exit is controlled by friction forces that limit the molecular mobility, but the nature of this friction is unknown. We introduce a method to probe the mobility of the tightly confined DNA by measuring DNA exit from phage phi29 capsids with optical tweezers. We measure extremely low initial exit velocity, a regime of exponentially increasing velocity, stochastic pausing that dominates the kinetics and large dynamic heterogeneity. Measurements with variable applied force provide evidence that the initial velocity is controlled by DNA-DNA sliding friction, consistent with a Frenkel-Kontorova model for nanoscale friction. We confirm several aspects of the ejection dynamics predicted by theoretical models. Features of the pausing suggest that it is connected to the phenomenon of 'clogging' in soft matter systems. Our results provide evidence that DNA-DNA friction and clogging control the DNA exit dynamics, but that this friction does not significantly affect DNA packaging.

Mechanical fatigue testing in silico: Dynamic evolution of material properties of nanoscale biological particles

Biological particles have evolved to possess mechanical characteristics necessary to carry out their functions. We developed a computational approach to "fatigue testing in silico", in which constant-amplitude cyclic loading is applied to a particle to explore its mechanobiology. We used this approach to describe dynamic evolution of nanomaterial properties and low-cycle fatigue in the thin spherical encapsulin shell, thick spherical Cowpea Chlorotic Mottle Virus (CCMV) capsid, and thick cylindrical microtubule (MT) fragment over 20 cycles of deformation. Changing structures and force-deformation curves enabled us to describe their damage-dependent biomechanics (strength, deformability, stiffness), thermodynamics (released and dissipated energies, enthalpy, and entropy) and material properties (toughness). Thick CCMV and MT particles experience material fatigue due to slow recovery and damage accumulation over 3-5 loading cycles; thin encapsulin shells show little fatigue due to rapid remodeling and limited damage. The results obtained challenge the existing paradigm: damage in biological particles is partially reversible owing to particle's partial recovery; fatigue crack may or may not grow with each loading cycle and may heal; and particles adapt to deformation amplitude and frequency to minimize the energy dissipated. Using crack size to quantitate damage is problematic as several cracks might form simultaneously in a particle. Dynamic evolution of strength, deformability, and stiffness, can be predicted by analyzing the cycle number (N) dependent damage, [Formula: see text] , where α is a power law and Nf is fatigue life. Fatigue testing in silico can now be used to explore damage-induced changes in the material properties of other biological particles. STATEMENT OF SIGNIFICANCE: Biological particles possess mechanical characteristics necessary to perform their functions. We developed "fatigue testing in silico" approach, which employes Langevin Dynamics simulations of constant-amplitude cyclic loading of nanoscale biological particles, to explore dynamic evolution of the mechanical, energetic, and material properties of the thin and thick spherical particles of encapsulin and Cowpea Chlorotic Mottle Virus, and the microtubule filament fragment. Our study of damage growth and fatigue development challenge the existing paradigm. Damage in biological particles is partially reversible as fatigue crack might heal with each loading cycle. Particles adapt to deformation amplitude and frequency to minimize energy dissipation. The evolution of strength, deformability, and stiffness, can be accurately predicted by analyzing the damage growth in particle structure.

Label-free microscopy for virus infections

Microscopy has been essential to elucidate micro- and nano-scale processes in space and time and has provided insights into cell and organismic functions. It is widely employed in cell biology, microbiology, physiology, clinical sciences and virology. While label-dependent microscopy, such as fluorescence microscopy, provides molecular specificity, it has remained difficult to multiplex in live samples. In contrast, label-free microscopy reports on overall features of the specimen at minimal perturbation. Here, we discuss modalities of label-free imaging at the molecular, cellular and tissue levels, including transmitted light microscopy, quantitative phase imaging, cryogenic electron microscopy or tomography and atomic force microscopy. We highlight how label-free microscopy is used to probe the structural organization and mechanical properties of viruses, including virus particles and infected cells across a wide range of spatial scales. We discuss the working principles of imaging procedures and analyses and showcase how they open new avenues in virology. Finally, we discuss orthogonal approaches that enhance and complement label-free microscopy techniques.

Role of DNA-DNA sliding friction and non-equilibrium dynamics in viral genome ejection and packaging

Many viruses eject their DNA via a nanochannel in the viral shell, driven by internal forces arising from the high-density genome packing. The speed of DNA exit is controlled by friction forces that limit the molecular mobility, but the nature of this friction is unknown. We introduce a method to probe the mobility of the tightly confined DNA by measuring DNA exit from phage phi29 capsids with optical tweezers. We measure extremely low initial exit velocity, a regime of exponentially increasing velocity, stochastic pausing that dominates the kinetics, and large dynamic heterogeneity. Measurements with variable applied force provide evidence that the initial velocity is controlled by DNA-DNA sliding friction, consistent with a Frenkel-Kontorova model for nanoscale friction. We confirm several aspects of the ejection dynamics predicted by theoretical models. Features of the pausing suggest it is connected to the phenomenon of "clogging" in soft-matter systems. Our results provide evidence that DNA-DNA friction and clogging control the DNA exit dynamics, but that this friction does not significantly affect DNA packaging.

Ring-stacked capsids of white spot syndrome virus and structural transitions with genome ejection

White spot syndrome virus (WSSV) is one of the largest DNA viruses and the major pathogen responsible for white spot syndrome in crustaceans. The WSSV capsid is critical for genome encapsulation and ejection and exhibits the rod-shaped and oval-shaped structures during the viral life cycle. However, the detailed architecture of the capsid and the structural transition mechanism remain unclear. Here, using cryo-electron microscopy (cryo-EM), we obtained a cryo-EM model of the rod-shaped WSSV capsid and were able to characterize its ring-stacked assembly mechanism. Furthermore, we identified an oval-shaped WSSV capsid from intact WSSV virions and analyzed the structural transition mechanism from the oval-shaped to rod-shaped capsids induced by high salinity. These transitions, which decrease internal capsid pressure, always accompany DNA release and mostly eliminate the infection of the host cells. Our results demonstrate an unusual assembly mechanism of the WSSV capsid and offer structural insights into the pressure-driven genome release.

Thermoresponsive C22 phage stiffness modulates the phage infectivity

Bacteriophages offer a sustainable alternative for controlling crop disease. However, the lack of knowledge on phage infection mechanisms makes phage-based biological control varying and ineffective. In this work, we interrogated the temperature dependence of the infection and thermo-responsive behavior of the C22 phage. This soilborne podovirus is capable of lysing Ralstonia solanacearum, causing bacterial wilt disease. We revealed that the C22 phage could better infect the pathogenic host cell when incubated at low temperatures (25, 30 °C) than at high temperatures (35, 40 °C). Measurement of the C22 phage stiffness revealed that the phage stiffness at low temperatures was 2–3 times larger than at high temperatures. In addition, the imaging results showed that more C22 phage particles were attached to the cell surface at low temperatures than at high temperatures, associating the phage stiffness and the phage attachment. The result suggests that the structure and stiffness modulation in response to temperature change improve infection, providing mechanistic insight into the C22 phage lytic cycle. Our study signifies the need to understand phage responses to the fluctuating environment for effective phage-based biocontrol implementation.

Antiviral Activity and Mechanisms of Seaweeds Bioactive Compounds on Enveloped Viruses-A Review

In the last decades, the interest in seaweed has significantly increased. Bioactive compounds from seaweed’s currently receive major attention from pharmaceutical companies as they express several interesting biological activities which are beneficial for humans. The structural diversity of seaweed metabolites provides diverse biological activities which are expressed through diverse mechanisms of actions. This review mainly focuses on the antiviral activity of seaweed’s extracts, highlighting the mechanisms of actions of some seaweed molecules against infection caused by different types of enveloped viruses: influenza, Lentivirus (HIV-1), Herpes viruses, and coronaviruses. Seaweed metabolites with antiviral properties can act trough different pathways by increasing the host’s defense system or through targeting and blocking virus replication before it enters host cells. Several studies have already established the large antiviral spectrum of seaweed’s bioactive compounds. Throughout this review, antiviral mechanisms and medical applications of seaweed’s bioactive compounds are analyzed, suggesting seaweed’s potential source of antiviral compounds for the formulation of novel and natural antiviral drugs.

The interferon-inducible GTPase MxB promotes capsid disassembly and genome release of herpesviruses

Host proteins sense viral products and induce defence mechanisms, particularly in immune cells. Using cell-free assays and quantitative mass spectrometry, we determined the interactome of capsid-host protein complexes of herpes simplex virus and identified the large dynamin-like GTPase myxovirus resistance protein B (MxB) as an interferon-inducible protein interacting with capsids. Electron microscopy analyses showed that cytosols containing MxB had the remarkable capability to disassemble the icosahedral capsids of herpes simplex viruses and varicella zoster virus into flat sheets of connected triangular faces. In contrast, capsids remained intact in cytosols with MxB mutants unable to hydrolyse GTP or to dimerize. Our data suggest that MxB senses herpesviral capsids, mediates their disassembly, and thereby restricts the efficiency of nuclear targeting of incoming capsids and/or the assembly of progeny capsids. The resulting premature release of viral genomes from capsids may enhance the activation of DNA sensors, and thereby amplify the innate immune responses.

Flavonoids Target Human Herpesviruses That Infect the Nervous System: Mechanisms of Action and Therapeutic Insights

Human herpesviruses (HHVs) are large DNA viruses with highly infectious characteristics. HHVs can induce lytic and latent infections in their host, and most of these viruses are neurotropic, with the capacity to generate severe and chronic neurological diseases of the peripheral nervous system (PNS) and central nervous system (CNS). Treatment of HHV infections based on strategies that include natural products-derived drugs is one of the most rapidly developing fields of modern medicine. Therefore, in this paper, we lend insights into the recent advances that have been achieved during the past five years in utilizing flavonoids as promising natural drugs for the treatment of HHVs infections of the nervous system such as alpha-herpesviruses (herpes simplex virus type 1, type 2, and varicella-zoster virus), beta-herpesviruses (human cytomegalovirus), and gamma-herpesviruses (Epstein–Barr virus and Kaposi sarcoma-associated herpesvirus). The neurological complications associated with infections induced by the reviewed herpesviruses are emphasized. Additionally, this work covers all possible mechanisms and pathways by which flavonoids induce promising therapeutic actions against the above-mentioned herpesviruses.

Intranuclear HSV-1 DNA ejection induces major mechanical transformations suggesting mechanoprotection of nucleus integrity

Maintaining nuclear integrity is essential to cell survival when exposed to mechanical stress. Herpesviruses, like most DNA and some RNA viruses, put strain on the nuclear envelope as hundreds of viral DNA genomes replicate and viral capsids assemble. It remained unknown, however, how nuclear mechanics is affected at the initial stage of herpesvirus infection-immediately after viral genomes are ejected into the nuclear space-and how nucleus integrity is maintained despite an increased strain on the nuclear envelope. With an atomic force microscopy force volume mapping approach on cell-free reconstituted nuclei with docked herpes simplex type 1 (HSV-1) capsids, we explored the mechanical response of the nuclear lamina and the chromatin to intranuclear HSV-1 DNA ejection into an intact nucleus. We discovered that chromatin stiffness, measured as Young's modulus, is increased by ∼14 times, while nuclear lamina underwent softening. Those transformations could be associated with a mechanism of mechanoprotection of nucleus integrity facilitating HSV-1 viral genome replication. Indeed, stiffening of chromatin, which is tethered to the lamina meshwork, helps to maintain nuclear morphology. At the same time, increased lamina elasticity, reflected by nucleus softening, acts as a "shock absorber," dissipating the internal mechanical stress on the nuclear membrane (located on top of the lamina wall) and preventing its rupture.

Reconstituted virus–nucleus system reveals mechanics of herpesvirus genome uncoating

The viral replication cycle is controlled by information transduced through both molecular and mechanical interactions. Viral infection mechanics remains largely unexplored, however, due to the complexity of cellular mechanical responses over the course of infection as well as a limited ability to isolate and probe these responses. Here, we develop an experimental system consisting of herpes simplex virus type 1 (HSV-1) capsids bound to isolated and reconstituted cell nuclei, which allows direct probing of capsid–nucleus mechanics with atomic force microscopy (AFM). Major mechanical transformations occur in the host nucleus when pressurised viral DNA ejects from HSV-1 capsids docked at the nuclear pore complexes (NPCs) on the nuclear membrane. This leads to structural rearrangement of the host chromosome, affecting its compaction. This in turn regulates viral genome replication and transcription dynamics as well as the decision between a lytic or latent course of infection. AFM probing of our reconstituted capsid–nucleus system provides high-resolution topographical imaging of viral capsid docking at the NPCs as well as force volume mapping of the infected nucleus surface, reflecting mechanical transformations associated with chromatin compaction and stiffness of nuclear lamina (to which chromatin is tethered). This experimental system provides a novel platform for investigation of virus–host interaction mechanics during viral genome penetration into the nucleus.

Role of HSV-1 Capsid Vertex-Specific Component (CVSC) and Viral Terminal DNA in Capsid Docking at the Nuclear Pore

Penetration of the viral genome into a host cell nucleus is critical for initiation of viral replication for most DNA viruses and a few RNA viruses. For herpesviruses, viral DNA ejection into a nucleus occurs when the capsid docks at the nuclear pore complex (NPC) basket with the correct orientation of the unique capsid portal vertex. It has been shown that capsid vertex-specific component (CVSC) proteins, which are located at the twelve vertices of the human herpes simplex virus type 1 (HSV-1) capsid, interact with nucleoporins (Nups) of NPCs. However, it remained unclear whether CVSC proteins determine capsid-to-NPC binding. Furthermore, it has been speculated that terminal DNA adjacent to the portal complex of DNA-filled C-capsids forms a structural motif with the portal cap (which retains DNA in the capsid), which mediates capsid-NPC binding. We demonstrate that terminal viral DNA adjacent to the portal proteins does not present a structural element required for capsid-NPC binding. Our data also show that level of CVSC proteins on the HSV-1 capsid affects level of NPC binding. To elucidate the capsid-binding process, we use an isolated, reconstituted cell nucleus system that recapitulates capsid-nucleus binding in vivo without interference from trafficking kinetics of capsids moving toward the nucleus. This allows binding of non-infectious capsid maturation intermediates with varying levels of vertex-specific components. This experimental system provides a platform for investigating virus-host interaction at the nuclear membrane.

Mechanical Capsid Maturation Facilitates the Resolution of Conflicting Requirements for Herpesvirus Assembly

Most viruses undergo a maturation process from a weakly self-assembled, noninfectious particle to a stable, infectious virion. For herpesviruses, this maturation process resolves several conflicting requirements: (i) assembly must be driven by weak, reversible interactions between viral particle subunits to reduce errors and minimize the energy of self-assembly, and (ii) the viral particle must be stable enough to withstand tens of atmospheres of DNA pressure resulting from its strong confinement in the capsid. With herpes simplex virus 1 (HSV-1) as a prototype of human herpesviruses, we demonstrated that this mechanical capsid maturation is mainly facilitated through capsid binding auxiliary protein UL25, orthologs of which are present in all herpesviruses. Through genetic manipulation of UL25 mutants of HSV-1 combined with the interrogation of capsid mechanics with atomic force microscopy nano-indentation, we suggested the mechanism of stepwise binding of distinct UL25 domains correlated with capsid maturation and DNA packaging. These findings demonstrate another paradigm of viruses as elegantly programmed nano-machines where an intimate relationship between mechanical and genetic information is preserved in UL25 architecture.

IMPORTANCE The minor capsid protein UL25 plays a critical role in the mechanical maturation of the HSV-1 capsid during virus assembly and is required for stable DNA packaging. We modulated the UL25 capsid interactions by genetically deleting different UL25 regions and quantifying the effect on mechanical capsid stability using an atomic force microscopy (AFM) nanoindentation approach. This approach revealed how UL25 regions reinforced the herpesvirus capsid to stably package and retain pressurized DNA. Our data suggest a mechanism of stepwise binding of two main UL25 domains timed with DNA packaging.

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.

Assembly of infectious Kaposi's sarcoma-associated herpesvirus progeny requires formation of a pORF19 pentamer

Herpesviruses cause severe diseases particularly in immunocompromised patients. Both genome packaging and release from the capsid require a unique portal channel occupying one of the 12 capsid vertices. Here, we report the 2.6 Å crystal structure of the pentameric pORF19 of the γ-herpesvirus Kaposi’s sarcoma-associated herpesvirus (KSHV) resembling the portal cap that seals this portal channel. We also present the structure of its β-herpesviral ortholog, revealing a striking structural similarity to its α- and γ-herpesviral counterparts despite apparent differences in capsid association. We demonstrate pORF19 pentamer formation in solution and provide insights into how pentamerization is triggered in infected cells. Mutagenesis in its lateral interfaces blocked pORF19 pentamerization and severely affected KSHV capsid assembly and production of infectious progeny. Our results pave the way to better understand the role of pORF19 in capsid assembly and identify a potential novel drug target for the treatment of herpesvirus-induced diseases.

In herpesviruses, genome packaging and release from the capsid require a unique portal channel. Here, the authors have resolved the crystal structure of a pentameric KSHV pORF19 assembly and find that it resembles the herpesviral portal cap and provides insights how the viral genome is retained within the capsid.

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 Ins and Outs of Herpesviral Capsids: Divergent Structures and Assembly Mechanisms across the Three Subfamilies

Human herpesviruses, classified into three subfamilies, are double-stranded DNA viruses that establish lifelong latent infections within most of the world’s population and can cause severe disease, especially in immunocompromised people. There is no cure, and current preventative and therapeutic options are limited. Therefore, understanding the biology of these viruses is essential for finding new ways to stop them. Capsids play a central role in herpesvirus biology. They are sophisticated vehicles that shelter the pressurized double-stranded-DNA genomes while ensuring their delivery to defined cellular destinations on the way in and out of the host cell. Moreover, the importance of capsids for multiple key steps in the replication cycle makes their assembly an attractive therapeutic target. Recent cryo-electron microscopy reconstructions of capsids from all three subfamilies of human herpesviruses revealed not only conserved features but also remarkable structural differences. Furthermore, capsid assembly studies have suggested subfamily-specific roles of viral capsid protein homologs. In this review, we compare capsid structures, assembly mechanisms, and capsid protein functions across human herpesvirus subfamilies, highlighting the differences.

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.

Entry of hepatitis B virus: going beyond NTCP to the nucleus

Hepatitis B virus (HBV) infection remains a major cause of liver diseases and hepatocellular carcinoma. HBV infection begins by low-affinity attachment to hepatocytes and subsequent binding with a specific receptor sodium taurocholate cotransporting polypeptide (NTCP) on sinusoidal-basolateral side of liver parenchymal cells. Following internalization with an unclear mechanism, HBV undergoes uncoating, capsid disassembling and culminates in delivering its genome into the nucleus and forms the covalently closed circular (ccc) DNA. In this review, we briefly summarize the current understanding of HBV entry and discuss some unanswered questions along the entry pathway beyond NTCP binding into the nucleus.

STING facilitates nuclear import of herpesvirus genome during infection

Once inside the host cell, DNA viruses must overcome the physical barrier posed by the nuclear envelope to establish a successful infection. The mechanism underlying this process remains unclear. Here, we show that the herpesvirus exploits the immune adaptor stimulator of interferon genes (STING) to facilitate nuclear import of the viral genome. Following the entry of the viral capsid into the cell, STING binds the viral capsid, mediates capsid docking to the nuclear pore complex via physical interaction, and subsequently enables accumulation of the viral genome in the nucleus. Silencing STING in human cytomegalovirus (HCMV)-susceptible cells inhibited nuclear import of the viral genome and reduced the ensuing viral gene expression. Overexpressing STING increased the host cell's susceptibility to HCMV and herpes simplex virus 1 by improving the nuclear delivery of viral DNA at the early stage of infection. These observations suggest that the proviral activity of STING is conserved and exploited by the herpesvirus family. Intriguingly, in monocytes, which act as latent reservoirs of HCMV, STING deficiency negatively regulated the establishment of HCMV latency and reactivation. Our findings identify STING as a proviral host factor regulating latency and reactivation of herpesviruses.

UL25 capsid binding facilitates mechanical maturation of the Herpesvirus capsid and allows retention of pressurized DNA

The maturation process that occurs in most viruses is evolutionarily driven as it resolves several conflicting virion assembly requirements. During herpesvirus assembly in a host cell nucleus, micron-long double-stranded herpes DNA is packaged into a nanometer-sized procapsid. This leads to strong confinement of the viral genome with resulting tens of atmospheres of intra-capsid DNA pressure. Yet, the procapsid is unstable due to weak, reversible interactions between its protein subunits, which ensures free energy minimization and reduces assembly errors. In this work we show that herpesviruses resolve these contradictory capsid requirements through a mechanical capsid maturation process facilitated by multi-functional auxiliary protein UL25. Through mechanical interrogation of herpes simplex virus type 1 (HSV-1) capsid with atomic force microscopy nano-indentation, we show that UL25 binding at capsid vertices post-assembly provides the critical capsid reinforcement required for stable DNA encapsidation; the absence of UL25 binding leads to capsid rupture. Furthermore, we demonstrate that gradual capsid reinforcement is a feasible maturation mechanism facilitated by progressive UL25 capsid binding, which is likely correlated with DNA packaging progression. This work provides insight into elegantly programmed viral assembly machinery where targeting of capsid assembly mechanics presents a new antiviral strategy that is resilient to development of drug resistance. Importance: Most viruses undergo a maturation process from a weakly assembled particle to a stable virion. Herpesvirus capsid undergoes mechanical maturation to withstand tens of atmospheres of DNA pressure. We demonstrate that this mechanical capsid maturation is mainly facilitated through binding of auxiliary protein UL25 in HSV-1 capsid vertices. We show that UL25 binding provides the critical capsid reinforcement required for stable DNA encapsidation. Our data also suggests that gradual capsid reinforcement by progressive UL25 binding is a feasible capsid maturation mechanism, correlated with DNA packaging progression.

Structural basis for genome packaging, retention, and ejection in human cytomegalovirus

How the human cytomegalovirus (HCMV) genome—the largest among human herpesviruses—is packaged, retained, and ejected remains unclear. We present the in situ structures of the symmetry-mismatched portal and the capsid vertex-specific components (CVSCs) of HCMV. The 5-fold symmetric 10-helix anchor—uncommon among known portals—contacts the portal-encircling DNA, which is presumed to squeeze the portal as the genome packaging proceeds. We surmise that the 10-helix anchor dampens this action to delay the portal reaching a “head-full” packaging state, thus facilitating the large genome to be packaged. The 6-fold symmetric turret, latched via a coiled coil to a helix from a major capsid protein, supports the portal to retain the packaged genome. CVSCs at the penton vertices—presumed to increase inner capsid pressure—display a low stoichiometry, which would aid genome retention. We also demonstrate that the portal and capsid undergo conformational changes to facilitate genome ejection after viral cell entry.

Human cytomegalovirus (HCMV) is the prototypical member of the β-herpesvirinae subfamily and the leading viral cause of congenital infections that can lead to birth defects and it can also cause life-threatening disease in immunocompromised individuals. Here, the authors present the in-situ cryo-EM structures of the symmetry-mismatched portal and the capsid vertex-specific components (CVSCs) of HCMV and discuss the mechanistic implications for genome package, retention and ejection.

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.

Berberine in Human Oncogenic Herpesvirus Infections and Their Linked Cancers

Human herpesviruses are known to induce a broad spectrum of diseases, ranging from common cold sores to cancer, and infections with some types of these viruses, known as human oncogenic herpesviruses (HOHVs), can cause cancer. Challenges with viral latency, recurrent infections, and drug resistance have generated the need for finding new drugs with the ability to overcome these barriers. Berberine (BBR), a naturally occurring alkaloid, is known for its multiple biological activities, including antiviral and anticancer effects. This paper comprehensively compiles all studies that have featured anti-HOHV properties of BBR along with promising preventive effects against the associated cancers. The mechanisms and pathways induced by BBR via targeting the herpesvirus life cycle and the pathogenesis of the linked malignancies are reviewed. Approaches to enhance the therapeutic efficacy of BBR and its use in clinical practice as an anti-herpesvirus drug are also discussed.

Scaling relation between genome length and particle size of viruses provides insights into viral life history

In terms of genome and particle sizes, viruses exhibit great diversity. With the discovery of several nucleocytoplasmic large DNA viruses (NCLDVs) and jumbo phages, the relationship between particle and genome sizes has emerged as an important criterion for understanding virus evolution. We use allometric scaling of capsid volume with the genome length of different groups of viruses to shed light on its relationship with virus life history. The allometric exponents for icosahedral dsDNA bacteriophages and NCDLVs were found to be 1 and 2, respectively, indicating that with increasing capsid size DNA packaging density remains the same in bacteriophages but decreases for NCLDVs. We argue that the exponents are largely shaped by their entry mechanism and capsid mechanical stability. We further show that these allometric size parameters are also intricately linked to the relative energy costs of translation and replication in viruses and can have further implications on viral life history.

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Capsid and genome size allometric exponent gives insights into viral life history

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The allometric exponent of NCLDVs is almost twice that of bacteriophages

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The exponent is largely shaped by the viral entry mechanism and capsid stability

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The relaxed genome size constraint allows large viruses to evolve greater autonomy

The Portal Vertex of KSHV Promotes Docking of Capsids at the Nuclear Pores

Kaposi's sarcoma-associated herpesvirus (KSHV) is a cancer-related herpesvirus. Like other herpesviruses, the KSHV icosahedral capsid includes a portal vertex, composed of 12 protein subunits encoded by open reading frame (ORF) 43, which enables packaging and release of the viral genome into the nucleus through the nuclear pore complex (NPC). Capsid vertex-specific component (CVSC) tegument proteins, which directly mediate docking at the NPCs, are organized on the capsid vertices and are enriched on the portal vertex. Whether and how the portal vertex is selected for docking at the NPC is unknown. Here, we investigated the docking of incoming ORF43-null KSHV capsids at the NPCs, and describe a significantly lower fraction of capsids attached to the nuclear envelope compared to wild-type (WT) capsids. Like WT capsids, nuclear envelope-associated ORF43-null capsids co-localized with different nucleoporins (Nups) and did not detach upon salt treatment. Inhibition of nuclear export did not alter WT capsid docking. As ORF43-null capsids exhibit lower extent of association with the NPCs, we conclude that although not essential, the portal has a role in mediating the interaction of the CVSC proteins with Nups, and suggest a model whereby WT capsids can dock at the nuclear envelope through a non-portal penton vertex, resulting in an infection 'dead end'.

Virus Mechanics under Molecular Crowding

Viruses avoid exposure of the viral genome to harmful agents with the help of a protective protein shell known as the capsid. A secondary effect of this protective barrier is that macromolecules that may be in high concentration on the outside cannot freely diffuse across it. Therefore, inside the cell and possibly even outside, the intact virus is generally under a state of osmotic stress. Viruses deal with this type of stress in various ways. In some cases, they might harness it for infection. However, the magnitude and influence of osmotic stress on virus physical properties remains virtually unexplored for single-stranded RNA viruses-the most abundant class of viruses. Here, we report on how a model system for the positive-sense RNA icosahedral viruses, brome mosaic virus (BMV), responds to osmotic pressure. Specifically, we study the mechanical properties and structural stability of BMV under controlled molecular crowding conditions. We show that BMV is mechanically reinforced under a small external osmotic pressure but starts to yield after a threshold pressure is reached. We explain this mechanochemical behavior as an effect of the molecular crowding on the entropy of the "breathing" fluctuation modes of the virus shell. The experimental results are consistent with the viral RNA imposing a small negative internal osmotic pressure that prestresses the capsid. Our findings add a new line of inquiry to be considered when addressing the mechanisms of viral disassembly inside the crowded environment of the cell.

Near-atomic cryo-electron microscopy structures of varicella-zoster virus capsids

Varicella-zoster virus (VZV) is a medically important human herpesvirus that causes chickenpox and shingles, but its cell-associated nature has hindered structure studies. Here we report the cryo-electron microscopy structures of purified VZV A-capsid and C-capsid, as well as of the DNA-containing capsid inside the virion. Atomic models derived from these structures show that, despite enclosing a genome that is substantially smaller than those of other human herpesviruses, VZV has a similarly sized capsid, consisting of 955 major capsid protein (MCP), 900 small capsid protein (SCP), 640 triplex dimer (Tri2) and 320 triplex monomer (Tri1) subunits. The VZV capsid has high thermal stability, although with relatively fewer intra- and inter-capsid protein interactions and less stably associated tegument proteins compared with other human herpesviruses. Analysis with antibodies targeting the N and C termini of the VZV SCP indicates that the hexon-capping SCP-the largest among human herpesviruses-uses its N-terminal half to bridge hexon MCP subunits and possesses a C-terminal flexible half emanating from the inner rim of the upper hexon channel into the tegument layer. Correlation of these structural features and functional observations provide insights into VZV assembly and pathogenesis and should help efforts to engineer gene delivery and anticancer vectors based on the currently available VZV vaccine.

Human Cytomegalovirus Primary Infection and Reactivation: Insights From Virion-Carried Molecules

Human cytomegalovirus (HCMV), a ubiquitous beta-herpesvirus, is able to establish lifelong latency after initial infection. Periodical reactivation occurs after immunosuppression, remaining a major cause of death in immunocompromised patients. HCMV has to reach a structural and functional balance with the host at its earliest entry. Virion-carried mediators are considered to play pivotal roles in viral adaptation into a new cellular environment upon entry. Additionally, one clear difference between primary infection and reactivation is the idea that virion-packaged factors are already formed such that those molecules can be used swiftly by the virus. In contrast, virion-carried mediators have to be transcribed and translated; thus, they are not readily available during reactivation. Hence, understanding virion-carried molecules helps to elucidate HCMV reactivation. In this article, the impact of virion-packaged molecules on viral structure, biological behavior, and viral life cycle will be reviewed.

CryoEM structure of the tegumented capsid of Epstein-Barr virus

Epstein-Barr virus (EBV) is the primary cause of infectious mononucleosis and has been shown to be closely associated with various malignancies. Here, we present a complete atomic model of EBV, including the icosahedral capsid, the dodecameric portal and the capsid-associated tegument complex (CATC). Our in situ portal from the tegumented capsid adopts a closed conformation with its channel valve holding the terminal viral DNA and with its crown region firmly engaged by three layers of ring-like dsDNA, which, together with the penton flexibility, effectively alleviates the capsid inner pressure placed on the portal cap. In contrast, the CATCs, through binding to the flexible penton vertices in a stoichiometric manner, accurately increase the inner capsid pressure to facilitate the pressure-driven genome delivery. Together, our results provide important insights into the mechanism by which the EBV capsid, portal, packaged genome and the CATCs coordinately achieve a pressure balance to simultaneously benefit both viral genome retention and ejection.

Pressurized DNA state inside herpes capsids-A novel antiviral target

Drug resistance in viruses represents one of the major challenges of healthcare. As part of an effort to provide a treatment that avoids the possibility of drug resistance, we discovered a novel mechanism of action (MOA) and specific compounds to treat all nine human herpesviruses and animal herpesviruses. The novel MOA targets the pressurized genome state in a viral capsid, "turns off" capsid pressure, and blocks viral genome ejection into a cell nucleus, preventing viral replication. This work serves as a proof-of-concept to demonstrate the feasibility of a new antiviral target-suppressing pressure-driven viral genome ejection-that is likely impervious to developing drug resistance. This pivotal finding presents a platform for discovery of a new class of broad-spectrum treatments for herpesviruses and other viral infections with genome-pressure-dependent replication. A biophysical approach to antiviral treatment such as this is also a vital strategy to prevent the spread of emerging viruses where vaccine development is challenged by high mutation rates or other evasion mechanisms.

Betaherpesvirus assembly and egress: Recent advances illuminate the path

The human betaherpesviruses, human cytomegalovirus (HCMV; species Human betaherpesvirus 5) and human herpesviruses 6A, 6B, and 7 (HHV-6A, -6B, and -7; species Human betaherpesviruses 6A, 6B, and 7) are highly prevalent and can cause severe disease in immune-compromised and immune-naive populations in well- and under-developed communities. Herpesvirus virion assembly is an intricate process that requires viral orchestration of host systems. In this review, we describe recent advances in some of the many cellular events relevant to assembly and egress of betaherpesvirus virions. These include modifications of host metabolic, immune, and autophagic/recycling systems. In addition, we discuss unique aspects of betaherpesvirus virion structure, virion assembly, and the cellular pathways employed during virion egress.