Penetration of nonenveloped viruses into the cytoplasm

Cholesterol-Dependent Membrane Deformation by Metastable Viral Capsids Facilitates Entry

Nonenveloped viruses employ unique entry mechanisms to breach and infect host cells. Understanding these mechanisms is crucial for developing antiviral strategies. Prevailing perspective suggests that nonenveloped viruses release membrane pore-forming peptides to breach host membranes. However, the precise involvement of the viral capsid in this entry remains elusive. Our study presents direct observations elucidating the dynamically distinctive steps through which metastable reovirus capsids disrupt host lipid membranes as they uncoat into partially hydrophobic intermediate particles. Using both live cells and model membrane systems, our key finding is that reovirus capsids actively deform and permeabilize lipid membranes in a cholesterol-dependent process. Unlike membrane pore-forming peptides, these metastable viral capsids induce more extensive membrane perturbations, including budding, bridging between adjacent membranes, and complete rupture. Notably, cholesterol enhances subviral particle adsorption, resulting in the formation of pores equivalent to the capsid size. This cholesterol dependence is attributed to the lipid condensing effect, particularly prominent at an intermediate cholesterol level. Furthermore, our results reveal a positive correlation between membrane disruption extent and efficiency of viral variants in establishing infection. This study unveils the crucial role of capsid-lipid interaction in nonenveloped virus entry, providing new insights into how cholesterol homeostasis influences virus infection dynamics.

Silencing RNA-Mediated Knockdown of IFITM3 Enhances Senecavirus A Replication

Senecavirus A (SVA) is a non-enveloped, positive sense, single-stranded RNA virus that causes vesicular diseases in pigs. Interferon-induced transmembrane 3 (IFITM3) is an interferon-stimulated gene (ISG) that exhibits broad antiviral activity. We investigated the role of IFITM3 in SVA replication. Both viral protein expression and supernatant virus titer were significantly increased when endogenous IFITM3 was knocked down by approximately 80% in human non-smallcell lung carcinoma cell line (NCI-H1299) compared to silencing RNA control. Interestingly, overexpression of exogenous IFITM3 in NCI-H1299 cells also significantly enhanced viral protein expression and virus titer compared to vector control, which was positively correlated with induction of autophagy mediated by IFITM3 overexpression. Overall, our results indicate an antiviral role of endogenous IFITM3 against SVA. The exact molecular mechanisms by which endogenous IFITM3 limits SVA replication remain to be determined in future studies.

Cholesterol-Dependent Membrane Deformation by Metastable Viral Capsids Facilitates Entry

Non-enveloped viruses employ unique entry mechanisms to breach and infect host cells. Understanding these mechanisms is crucial for developing antiviral strategies. Prevailing perspective suggests that non-enveloped viruses release membrane lytic peptides to breach host membranes. However, the precise involvement of the viral capsid in this entry remains elusive. Our study presents direct observations elucidating the dynamically distinctive steps through which metastable reovirus capsids disrupt host lipid membranes as they uncoat into partially hydrophobic intermediate particles. Using both live cells and model membrane systems, our key finding is that reovirus capsids actively deform and permeabilize lipid membranes in a cholesterol-dependent process. Unlike membrane lytic peptides, these metastable viral capsids induce more extensive membrane perturbations, including budding, bridging between adjacent membranes, and complete rupture. Notably, cholesterol enhances subviral particle adsorption, resulting in the formation of pores equivalent to the capsid size. This cholesterol dependence is attributed to the lipid condensing effect, particularly prominent at intermediate cholesterol level. Furthermore, our results reveal a positive correlation between membrane disruption extent and efficiency of viral variants in establishing infection. This study unveils the crucial role of capsid-lipid interaction in non-enveloped virus entry, providing new insights into how cholesterol homeostasis influences virus infection dynamics.

The atlastin ER morphogenic proteins promote formation of a membrane penetration site during non-enveloped virus entry

During entry, non-enveloped viruses penetrate a host membrane to cause infection, although how this is accomplished remains enigmatic. Polyomaviruses (PyVs) are non-enveloped DNA viruses that penetrate the endoplasmic reticulum (ER) membrane to reach the cytosol en route to the nucleus for infection. To penetrate the ER membrane, the prototype PyV simian virus 40 (SV40) induces formation of ER-escape sites, called foci, composed of repeating units of multi-tubular ER junctions where the virus is thought to exit. How SV40 triggers formation of the ER-foci harboring these multi-tubular ER junctions is unclear. Here, we show that the ER morphogenic atlastin 2 (ATL2) and ATL3 membrane proteins play critical roles in SV40 infection. Mechanistically, ATL3 mobilizes to the ER-foci where it deploys its GTPase-dependent membrane fusion activity to promote formation of multi-tubular ER junctions within the ER-foci. ATL3 also engages an SV40-containing membrane penetration complex. By contrast, ATL2 does not reorganize to the ER-foci. Instead, it supports the reticular ER morphology critical for the integrity of the ATL3-dependent membrane complex. Our findings illuminate how two host factors play distinct roles in the formation of an essential membrane penetration site for a non-enveloped virus. IMPORTANCE Membrane penetration by non-enveloped viruses, a critical infection step, remains enigmatic. The non-enveloped PyV simian virus 40 (SV40) penetrates the endoplasmic reticulum (ER) membrane to reach the cytosol en route for infection. During ER-to-cytosol membrane penetration, SV40 triggers formation of ER-associated structures (called ER-foci) that function as the membrane penetration sites. Here, we discover a role of the ATL ER membrane proteins-known to shape the ER morphology-during SV40-induced ER-foci formation. These findings illuminate how a non-enveloped virus hijacks host components to construct a membrane penetration structure.

Interferon-Inducible Transmembrane Protein 3 (IFITM3) Restricts Rotavirus Infection

Rotavirus (RV) is a non-enveloped icosahedral virus with an 11-segment double-stranded RNA genome, belonging to the family of rotaviruses. RV is one of the pathogens causing diarrhea in infants and young animals, and it induces the production of type I interferons (IFNs), which can trigger antiviral function by inducing the production of interferon-stimulated genes (ISGs). Although IFITM3, an ISG localizing to late endosomes, can limit many viral infections, whether or not it restricts the infection of RV is still unknown. Therefore, we attempted to determine whether IFITM3 also restricts RV infection by using over-expression and knockout cell strains. It was found that IFITM3-expressing cell strains were less susceptible to RV infection, as the replication of RV in over-expressing cells was significantly less than in control group cells. Correspondingly, IFITM3-knockout cells were significantly susceptible compared to the normal cells. Furthermore, the IFN-induced antiviral effect was significantly attenuated in the absence of IFITM3, and IFITM3 delayed RV escape from endosomes in the presence of IFITM3, suggesting that endogenous IFITM3 is of great importance in type I IFN-mediated antiviral responses and may restrict infection by affecting the function of the late endosomal compartment. In conclusion, these data provide the first evidence that IFITM3 limits RV infection in vitro and delays RV escape from late endosomes into the cytoplasm.

Reovirus infection is regulated by NPC1 and endosomal cholesterol homeostasis

Cholesterol homeostasis is required for the replication of many viruses, including Ebola virus, hepatitis C virus, and human immunodeficiency virus-1. Niemann-Pick C1 (NPC1) is an endosomal-lysosomal membrane protein involved in cholesterol trafficking from late endosomes and lysosomes to the endoplasmic reticulum. We identified NPC1 in CRISPR and RNA interference screens as a putative host factor for infection by mammalian orthoreovirus (reovirus). Following internalization via clathrin-mediated endocytosis, the reovirus outer capsid is proteolytically removed, the endosomal membrane is disrupted, and the viral core is released into the cytoplasm where viral transcription, genome replication, and assembly take place. We found that reovirus infection is significantly impaired in cells lacking NPC1, but infection is restored by treatment of cells with hydroxypropyl-β-cyclodextrin, which binds and solubilizes cholesterol. Absence of NPC1 did not dampen infection by infectious subvirion particles, which are reovirus disassembly intermediates that bypass the endocytic pathway for infection of target cells. NPC1 is not required for reovirus attachment to the plasma membrane, internalization into cells, or uncoating within endosomes. Instead, NPC1 is required for delivery of transcriptionally active reovirus core particles from endosomes into the cytoplasm. These findings suggest that cholesterol homeostasis, ensured by NPC1 transport activity, is required for reovirus penetration into the cytoplasm, pointing to a new function for NPC1 and cholesterol homeostasis in viral infection.

Ruminal Phages - A Review

The rumen ecosystem is a complex and dynamic environment, which hosts microorganisms including archaea, bacteria, protozoa, fungi, and viruses. These microorganisms interact with each other, altering the ruminal environment and substrates that will be available for the host digestion and metabolism. Viruses can infect the host and other microorganisms, which can drive changes in microorganisms' lysis rate, substrate availability, nutrient recycling, and population structure. The lysis of ruminal microorganisms' cells by viruses can release enzymes that enhance feedstuff fermentation, which may increase dietary nutrient utilization and feed efficiency. However, negative effects associated to viruses in the gastrointestinal tract have also been reported, in some cases, disrupting the dynamic stability of the ruminal microbiome, which can result in gastrointestinal dysfunctions. Therefore, the objective of this review is to summarize the current knowledge on ruminal virome, their interaction with other components of the microbiome and the effects on animal nutrition.

Lunapark-dependent formation of a virus-induced ER exit site contains multi-tubular ER junctions that promote viral ER-to-cytosol escape

Viruses rearrange host membranes to support different entry steps. Polyomavirus simian virus 40 (SV40) reorganizes the endoplasmic reticulum (ER) membrane to generate focus structures that enable virus ER-to-cytosol escape, a decisive infection step. The molecular architecture of the ER exit site that might illuminate why it is ideally suited for membrane penetration is unknown. Here 3D focused ion beam scanning electron microscopy (FIB-SEM) reconstruction reveals that the ER focus structure consists of multi-tubular ER junctions where SV40 preferentially localizes, suggesting that tubular branch points are virus ER-to-cytosol penetration sites. Functional analysis demonstrates that lunapark-an ER membrane protein that typically stabilizes three-way ER junctions-relocates to the ER foci, where it supports focus formation, leading to SV40 ER escape and infection. Our results reveal how a virus repurposes the activity of an ER membrane protein to form a virus-induced ER substructure required for membrane escape and suggest that ER tubular junctions are vulnerable sites exploited by viruses for membrane penetration.

Treading a HOSTile path: Mapping the dynamic landscape of host cell-rotavirus interactions to explore novel host-directed curative dimensions

Viruses are intracellular pathogens and are dependent on host cellular resources to carry out their cycles of perpetuation. Obtaining an integrative view of host-virus interaction is of utmost importance to understand the complex and dynamic interplay between viral components and host machineries. Besides its obvious scholarly significance, a comprehensive host-virus interaction profile also provides a platform where from host determinants of pro-viral and antiviral importance can be identified and further be subjected to therapeutic intervention. Therefore, adjunct to conventional methods of prophylactic vaccination and virus-directed antivirals, this host-targeted antiviral approach holds promising therapeutic potential. In this review, we present a comprehensive landscape of host cellular reprogramming in response to infection with rotavirus (RV) which causes profuse watery diarrhea in neonates and infants. In addition, an emphasis is given on how host determinants are either usurped or subverted by RV in course of infection and how therapeutic manipulation of specific host factors can effectively modulate the RV life cycle.

Bluetongue virus capsid protein VP5 perforates membranes at low endosomal pH during viral entry

Bluetongue virus (BTV) is a non-enveloped virus and causes substantial morbidity and mortality in ruminants such as sheep. Fashioning a receptor-binding protein (VP2) and a membrane penetration protein (VP5) on the surface, BTV releases its genome-containing core (VP3 and VP7) into the host cell cytosol after perforation of the endosomal membrane. Unlike enveloped ones, the entry mechanisms of non-enveloped viruses into host cells remain poorly understood. Here we applied single-particle cryo-electron microscopy, cryo-electron tomography and structure-guided functional assays to characterize intermediate states of BTV cell entry in endosomes. Four structures of BTV at the resolution range of 3.4-3.9 Å show the different stages of structural rearrangement of capsid proteins on exposure to low pH, including conformational changes of VP5, stepwise detachment of VP2 and a small shift of VP7. In detail, sensing of the low-pH condition by the VP5 anchor domain triggers three major VP5 actions: projecting the hidden dagger domain, converting a surface loop to a protonated β-hairpin that anchors VP5 to the core and stepwise refolding of the unfurling domains into a six-helix stalk. Cryo-electron tomography structures of BTV interacting with liposomes show a length decrease of the VP5 stalk from 19.5 to 15.5 nm after its insertion into the membrane. Our structures, functional assays and structure-guided mutagenesis experiments combined indicate that this stalk, along with dagger domain and the WHXL motif, creates a single pore through the endosomal membrane that enables the viral core to enter the cytosol. Our study unveils the detailed mechanisms of BTV membrane penetration and showcases general methods to study cell entry of other non-enveloped viruses.

Atomistic dynamics of a viral infection process: Release of membrane lytic peptides from a non-enveloped virus

Molecular simulations have played an instrumental role in uncovering the structural dynamics and physical properties of virus capsids. In this work, we move beyond equilibrium physicochemical characterization of a virus system to study a stage of the infection process that is required for viral proliferation. Despite many biochemical and functional studies, the molecular mechanism of host cell entry by non-enveloped viruses remains largely unresolved. Flock House virus (FHV) is a model system for non-enveloped viruses and is the subject of the current study. FHV infects through the acid-dependent endocytic pathway, where low pH triggers externalization of membrane-disrupting (γ) peptides from the capsid interior. Using all-atom equilibrium and enhanced sampling simulations, the mechanism and energetics of γ peptide liberation and the effect of pH on this process are investigated. Our computations agree with experimental findings and reveal nanoscopic details regarding the pH control mechanism, which are not readily accessible in experiments.

Real-Time Imaging of Polioviral RNA Translocation across a Membrane

Genome transfer from a virus into a cell is a critical early step in viral replication. Enveloped viruses achieve the delivery of their genomes into the cytoplasm by merging the viral membrane with the cellular membrane via a conceptually simple mechanism called membrane fusion. In contrast, genome translocation mechanisms in nonenveloped viruses, which lack viral membranes, remain poorly understood. Although cellular assays provide useful information about cell entry and genome release, it is difficult to obtain detailed mechanistic insights due both to the inherent technical difficulties associated with direct visualization of these processes and to the prevalence of nonproductive events in cellular assays performed at a very high multiplicity of infection. To overcome these issues, we developed an in vitro single-particle fluorescence assay to characterize genome release from a nonenveloped virus (poliovirus) in real time using a tethered receptor-decorated liposome system. Our results suggest that poliovirus genome release is a complex process that consists of multiple rate-limiting steps. Interestingly, we found that the addition of exogenous wild-type capsid protein VP4, but not mutant VP4, enhanced the efficiency of genome translocation. These results, together with prior structural analysis, suggest that VP4 interacts with RNA directly and forms a protective, membrane-spanning channel during genome translocation. Furthermore, our data indicate that VP4 dynamically interacts with RNA, rather than forming a static tube for RNA translocation. This study provides new insights into poliovirus genome translocation and offers a cell-free assay that can be utilized broadly to investigate genome release processes in other nonenveloped viruses.

SARS-CoV-2 in environmental perspective: Occurrence, persistence, surveillance, inactivation and challenges

The unprecedented global spread of the severe acute respiratory syndrome (SARS) caused by SARS-CoV-2 is depicting the distressing pandemic consequence on human health, economy as well as ecosystem services. So far novel coronavirus (CoV) outbreaks were associated with SARS-CoV-2 (2019), middle east respiratory syndrome coronavirus (MERS-CoV, 2012), and SARS-CoV-1 (2003) events. CoV relates to the enveloped family of Betacoronavirus (βCoV) with positive-sense single-stranded RNA (+ssRNA). Knowing well the persistence, transmission, and spread of SARS-CoV-2 through proximity, the faecal-oral route is now emerging as a major environmental concern to community transmission. The replication and persistence of CoV in the gastrointestinal (GI) tract and shedding through stools is indicating a potential transmission route to the environment settings. Despite of the evidence, based on fewer reports on SARS-CoV-2 occurrence and persistence in wastewater/sewage/water, the transmission of the infective virus to the community is yet to be established. In this realm, this communication attempted to review the possible influx route of the enteric enveloped viral transmission in the environmental settings with reference to its occurrence, persistence, detection, and inactivation based on the published literature so far. The possibilities of airborne transmission through enteric virus-laden aerosols, environmental factors that may influence the viral transmission, and disinfection methods (conventional and emerging) as well as the inactivation mechanism with reference to the enveloped virus were reviewed. The need for wastewater epidemiology (WBE) studies for surveillance as well as for early warning signal was elaborated. This communication will provide a basis to understand the SARS-CoV-2 as well as other viruses in the context of the environmental engineering perspective to design effective strategies to counter the enteric virus transmission and also serves as a working paper for researchers, policy makers and regulators.

Vesicular trafficking permits evasion of cGAS/STING surveillance during initial human papillomavirus infection

Oncogenic human papillomaviruses (HPVs) replicate in differentiating epithelium, causing 5% of cancers worldwide. Like most other DNA viruses, HPV infection initiates after trafficking viral genome (vDNA) to host cell nuclei. Cells possess innate surveillance pathways to detect microbial components or physiological stresses often associated with microbial infections. One of these pathways, cGAS/STING, induces IRF3-dependent antiviral interferon (IFN) responses upon detection of cytosolic DNA. Virion-associated vDNA can activate cGAS/STING during initial viral entry and uncoating/trafficking, and thus cGAS/STING is an obstacle to many DNA viruses. HPV has a unique vesicular trafficking pathway compared to many other DNA viruses. As the capsid uncoats within acidic endosomal compartments, minor capsid protein L2 protrudes across vesicular membranes to facilitate transport of vDNA to the Golgi. L2/vDNA resides within the Golgi lumen until G2/M, whereupon vesicular L2/vDNA traffics along spindle microtubules, tethering to chromosomes to access daughter cell nuclei. L2/vDNA-containing vesicles likely remain intact until G1, following nuclear envelope reformation. We hypothesize that this unique vesicular trafficking protects HPV from cGAS/STING surveillance. Here, we investigate cGAS/STING responses to HPV infection. DNA transfection resulted in acute cGAS/STING activation and downstream IFN responses. In contrast, HPV infection elicited minimal cGAS/STING and IFN responses. To determine the role of vesicular trafficking in cGAS/STING evasion, we forced premature viral penetration of vesicular membranes with membrane-perturbing cationic lipids. Such treatment renders a non-infectious trafficking-defective mutant HPV infectious, yet susceptible to cGAS/STING detection. Overall, HPV evades cGAS/STING by its unique subcellular trafficking, a property that may contribute to establishment of infection.

Backbone Free Energy Estimator Applied to Viral Glycoproteins

Earlier analysis of the Protein Data Bank derived the distribution of rotations from the plane of a protein hydrogen bond donor peptide group to the plane of its acceptor peptide group. The quasi Boltzmann formalism of Pohl-Finkelstein is employed to estimate free energies of protein elements with these hydrogen bonds, pinpointing residues with a high propensity for conformational change. This is applied to viral glycoproteins as well as capsids, where the 90th+ percentiles of free energies determine residues that correlate well with viral fusion peptides and other functional domains in known cases and thus provide a novel method for predicting these sites of importance as antiviral drug or vaccine targets in general. The method is implemented at https://bion-server.au.dk/hbonds/ from an uploaded Protein Data Bank file.

Changes in Membrane Dielectric Properties of Porcine Kidney Cells Provide Insight into the Antiviral Activity of Glycine

The ability to monitor the status and progression of viral infections is important for development and screening of new antiviral drugs. Previous research illustrated that the osmolyte glycine (Gly) reduced porcine parvovirus (PPV) infection in porcine kidney (PK-13) cells by stabilizing the capsid protein and preventing virus capsid assembly into viable virus particles. Dielectrophoresis (DEP) was examined herein as a noninvasive, electric field- and frequency-dependent tool for real-time monitoring of PK-13 cell responses to obtain information about membrane barrier functionality and polarization. DEP responses of PK-13 cells were compared to those of PPV-infected cells in the absence and presence of the osmolyte glycine. With infection progression, PK-13 DEP spectra shifted toward lower frequencies, reducing crossover frequencies (f_CO). The spherical single-shell model was used to extract PK-13 cell dielectric properties. Upon PPV infection, specific membrane capacitance increased over the time progression of virus attachment, penetration, and capsid protein production and assembly. Following glycine treatment, the DEP spectra displayed attenuated f_CO and specific membrane capacitance values shifted back toward uninfected PK-13 cell values. These results suggest that DEP can be used to noninvasively monitor the viral infection cycle and screen antiviral compounds. DEP can augment traditional tools by elucidating membrane polarization changes related to drug mechanisms that interrupt the virus infection cycle.

Structural and cellular biology of adeno-associated virus attachment and entry

Adeno-associated virus (AAV) is a nonenveloped, ssDNA virus in the parvovirus family, which has become one of the leading candidate vectors for human gene therapy. AAV has been studied extensively to identify host cellular factors involved in infection, as well as to identify capsid variants that confer clinically favorable transduction profiles ex vivo and in vivo. Recent advances in technology have allowed for direct genetic approaches to be used to more comprehensively characterize host factors required for AAV infection and allowed for identification of a critical multi-serotype receptor, adeno-associated virus receptor (AAVR). In this chapter, we will discuss the interactions of AAV with its glycan and proteinaceous receptors and describe the host and viral components involved in AAV entry, which requires cellular attachment, endocytosis, trafficking to the trans-Golgi network and nuclear import. AAV serves as a paradigm for entry of nonenveloped viruses. Furthermore, we will discuss the potential of utilizing our increased understanding of virus-host interactions during AAV entry to develop better AAV-based therapeutics, with a focus on host factors and capsid interactions involved in in vivo tropism.

Structural Dynamics of Nonenveloped Virus Disassembly Intermediates

The stability of icosahedral viruses is crucial for protecting the viral genome during transit; however, successful infection requires eventual disassembly of the capsid. A comprehensive understanding of how stable, uniform icosahedrons disassemble remains elusive, mainly due to the complexities involved in isolating transient intermediates. We utilized incremental heating to systematically characterize the disassembly pathway of a model nonenveloped virus and identified an intriguing link between virus maturation and disassembly. Further, we isolated and characterized two intermediates by cryo-electron microscopy and three-dimensional reconstruction, without imposing icosahedral symmetry. The first intermediate displayed a series of major, asymmetric alterations, whereas the second showed that the act of genome release, through the 2-fold axis, is actually confined to a small section on the capsid. Our study thus presents a comprehensive structural analysis of nonenveloped virus disassembly and emphasizes the asymmetric nature of programmed conformational changes.IMPORTANCE Disassembly or uncoating of an icosahedral capsid is a crucial step during infection by nonenveloped viruses. However, the dynamic and transient nature of the disassembly process makes it challenging to isolate intermediates in a temporal, stepwise manner for structural characterization. Using controlled, incremental heating, we isolated two disassembly intermediates: "eluted particles" and "puffed particles" of an insect nodavirus, Flock House virus (FHV). Cryo-electron microscopy and three-dimensional reconstruction of the FHV disassembly intermediates indicated that disassembly-related conformational alterations are minimally global and largely local, leading to asymmetry in the particle and eventual genome release without complete disintegration of the icosahedron.

How non-enveloped viruses hijack host machineries to cause infection

Viruses must navigate the complex endomembranous network of the host cell to cause infection. In the case of a non-enveloped virus that lacks a surrounding lipid bilayer, endocytic uptake from the plasma membrane is not sufficient to cause infection. Instead, the virus must travel within organelle membranes to reach a specific cellular destination that supports exposure or arrival of the virus to the cytosol. This is achieved by viral penetration across a host endomembrane, ultimately enabling entry of the virus into the nucleus to initiate infection. In this review, we discuss the entry mechanisms of three distinct non-enveloped DNA viruses-adenovirus (AdV), human papillomavirus (HPV), and polyomavirus (PyV)-highlighting how each exploit different intracellular transport machineries and membrane penetration apparatus associated with the endosome, Golgi, and endoplasmic reticulum (ER) membrane systems to infect a host cell. These processes not only illuminate a highly-coordinated interplay between non-enveloped viruses and their host, but may provide new strategies to combat non-enveloped virus-induced diseases.

Bag2 Is a Component of a Cytosolic Extraction Machinery That Promotes Membrane Penetration of a Nonenveloped Virus

During entry, the nonenveloped polyomavirus (PyV) simian virus 40 (SV40) traffics from the cell surface to the endoplasmic reticulum (ER), where it penetrates the ER membrane to reach the cytosol; the virus is then transported into the nucleus to cause infection. Although a coherent understanding of SV40's host entry is emerging, how the virus is ejected from the ER into the cytosol remains mysterious. Our previous analyses revealed that the cytosolic Hsc70-SGTA-Hsp105 complex binds to SV40 and extracts it from the ER into the cytosol. We now report that the nucleotide exchange factor (NEF) Bag2 stimulates SV40 release from Hsc70, thereby enabling successful virus arrival at the cytosol, which leads to infection. Hsp105, another NEF of Hsc70, displays a function overlapping that of Bag2, underscoring the importance of this release reaction. Our findings identify a new component of an extraction machinery essential during membrane penetration of a nonenveloped virus and provide further mechanistic insights into this process.IMPORTANCE How a nonenveloped virus penetrates a biological membrane to cause infection is a mystery. For the nonenveloped polyomavirus SV40, transport across the ER membrane to reach the cytosol is an essential virus infection step. Here, we identify a novel component of a cytosolic Hsc70-dependent chaperone complex called Bag2 that extracts SV40 from the ER into the cytosol. Bag2 does this by triggering SV40 release from Hsc70, thus ensuring that the virus reaches the cytosol en route for productive infection.

Programmable In Vitro Coencapsidation of Guest Proteins for Intracellular Delivery by Virus-like Particles

Bioinspired self-sorting and self-assembling systems using engineered versions of natural protein cages are being developed for biocatalysis and therapeutic delivery. The packaging and intracellular delivery of guest proteins is of particular interest for both in vitro and in vivo cell engineering. However, there is a lack of bionanotechnology platforms that combine programmable guest protein encapsidation with efficient intracellular uptake. We report a minimal peptide anchor for in vivo self-sorting of cargo-linked capsomeres of murine polyomavirus (MPyV) that enables controlled encapsidation of guest proteins by in vitro self-assembly. Using Förster resonance energy transfer, we demonstrate the flexibility in this system to support coencapsidation of multiple proteins. Complementing these ensemble measurements with single-particle analysis by super-resolution microscopy shows that the stochastic nature of coencapsidation is an overriding principle. This has implications for the design and deployment of both native and engineered self-sorting encapsulation systems and for the assembly of infectious virions. Taking advantage of the encoded affinity for sialic acids ubiquitously displayed on the surface of mammalian cells, we demonstrate the ability of self-assembled MPyV virus-like particles to mediate efficient delivery of guest proteins to the cytosol of primary human cells. This platform for programmable coencapsidation and efficient cytosolic delivery of complementary biomolecules therefore has enormous potential in cell engineering.

Conserved Surface Residues on the Feline Calicivirus Capsid Are Essential for Interaction with Its Receptor Feline Junctional Adhesion Molecule A (fJAM-A)

Host cell surface receptors are required for attachment, binding, entry, and infection by nonenveloped viruses. Receptor binding can induce conformational changes in the viral capsid and/or the receptor that couple binding with downstream events in the virus life cycle (intracellular signaling, endocytosis and trafficking, and membrane penetration). Virus-receptor interactions also influence viral spread and pathogenicity. The interaction between feline calicivirus (FCV) and its receptor, feline junctional adhesion molecule A (fJAM-A), on host cells is required for infection and induces irreversible, inactivating conformational changes in the capsid of some viral strains. Cryoelectron microscopy (cryo-EM) structures of FCV bound to fJAM-A showed several possible virus-receptor interactions. However, the specific residues on the viral capsid required for binding are not known. Capsid residues that may be involved in postbinding events have been implicated by isolation of soluble receptor-resistant (srr) mutants in which changes in the capsid protein sequence change the capacity of such srr mutants to be inactivated upon incubation with soluble fJAM-A. To clarify which residues on the surface of FCV are required for its interaction with fJAM-A and to potentially identify residues required for postreceptor binding events, we used the existing atomic-resolution structures of FCV and the FCV-fJAM-A cryo-EM structures to select 14 capsid residues for mutation and preparation of recombinant viral capsids. Using this approach, we identified residues on the FCV capsid that are required for fJAM-A binding and other residues that are not required for binding but are required for infection that are likely important for subsequent postbinding events.IMPORTANCE Feline calicivirus (FCV) is a common cause of mild upper respiratory disease in cats. Some FCV isolates can cause virulent systemic disease. The genetic determinants of virulence for FCV are unknown. We previously found that virulent FCV isolates have faster in vitro growth kinetics than less virulent isolates. Differences in viral growth in vitro may correlate with differences in virulence. Here, we investigated the roles of specific FCV capsid residues on the receptor-virus interaction and viral growth in vitro We show that the capsid protein genes of the virulent FCV-5 isolate determine its faster in vitro growth kinetics compared to those of the nonvirulent FCV-Urbana infectious clone. We also identified residues on the capsid VP1 protein that are important for receptor binding or for steps subsequent to receptor binding. Our data provide further insight into the specific molecular interactions between fJAM-A and the FCV capsid that regulate binding and infectious entry.

Activation of PI3K, Akt, and ERK during early rotavirus infection leads to V-ATPase-dependent endosomal acidification required for uncoating

The cellular PI3K/Akt and/or MEK/ERK signaling pathways mediate the entry process or endosomal acidification during infection of many viruses. However, their roles in the early infection events of group A rotaviruses (RVAs) have remained elusive. Here, we show that late-penetration (L-P) human DS-1 and bovine NCDV RVA strains stimulate these signaling pathways very early in the infection. Inhibition of both signaling pathways significantly reduced production of viral progeny due to blockage of virus particles in the late endosome, indicating that neither of the two signaling pathways is involved in virus trafficking. However, immunoprecipitation assays using antibodies specific for pPI3K, pAkt, pERK and the subunit E of the V-ATPase co-immunoprecipitated the V-ATPase in complex with pPI3K, pAkt, and pERK. Moreover, Duolink proximity ligation assay revealed direct association of the subunit E of the V-ATPase with the molecules pPI3K, pAkt, and pERK, indicating that both signaling pathways are involved in V-ATPase-dependent endosomal acidification. Acidic replenishment of the medium restored uncoating of the RVA strains in cells pretreated with inhibitors specific for both signaling pathways, confirming the above results. Isolated components of the outer capsid proteins, expressed as VP4-VP8* and VP4-VP5* domains, and VP7, activated the PI3K/Akt and MEK/ERK pathways. Furthermore, psoralen-UV-inactivated RVA and CsCl-purified RVA triple-layered particles triggered activation of the PI3K/Akt and MEK/ERK pathways, confirming the above results. Our data demonstrate that multistep binding of outer capsid proteins of L-P RVA strains with cell surface receptors phosphorylates PI3K, Akt, and ERK, which in turn directly interact with the subunit E of the V-ATPase to acidify the late endosome for uncoating of RVAs. This study provides a better understanding of the RVA-host interaction during viral uncoating, which is of importance for the development of strategies aiming at controlling or preventing RVA infections.

Viral particles must transport their genome into the cytoplasm or the nucleus of host cells to initiate successful infection. Knowledge of how viruses may pirate host cell signaling cascades or molecules to promote their own replication can facilitate the development of antiviral drugs. Group A rotavirus (RVA) is a major etiological agent of acute gastroenteritis in young children and the young of various mammals. RVA enters cells by a complex multistep process. However, the cellular signaling cascades or molecules that facilitate these processes are incompletely understood. Here, we demonstrate that infection with late-penetration RVA strains results in phosphorylation of PI3K, Akt, and ERK signaling molecules at an early stage of infection, a process mediated by the multistep binding of RVAs outer capsid proteins. Specific inhibitors for PI3K/Akt and MEK/ERK signaling pathways trap the viral particles in late endosome, and acidic replenishment restores and releases them. Moreover, the RVA-induced phosphorylated PI3K, Akt, and ERK directly interact with the subunit E of the V-ATPase proton pump, required for endosomal acidification and RVA uncoating. Understanding how RVA-induced early activation of cellular signaling molecules mediates the V-ATPase-dependent endosomal acidification required for uncoating of viral particles opens up opportunities for targeted interventions against rotavirus entry.

The Neuronal Gene Arc Encodes a Repurposed Retrotransposon Gag Protein that Mediates Intercellular RNA Transfer

The neuronal gene Arc is essential for long-lasting information storage in the mammalian brain, mediates various forms of synaptic plasticity, and has been implicated in neurodevelopmental disorders. However, little is known about Arc's molecular function and evolutionary origins. Here, we show that Arc self-assembles into virus-like capsids that encapsulate RNA. Endogenous Arc protein is released from neurons in extracellular vesicles that mediate the transfer of Arc mRNA into new target cells, where it can undergo activity-dependent translation. Purified Arc capsids are endocytosed and are able to transfer Arc mRNA into the cytoplasm of neurons. These results show that Arc exhibits similar molecular properties to retroviral Gag proteins. Evolutionary analysis indicates that Arc is derived from a vertebrate lineage of Ty3/gypsy retrotransposons, which are also ancestors to retroviruses. These findings suggest that Gag retroelements have been repurposed during evolution to mediate intercellular communication in the nervous system.

Cystic Fibrosis, Cystic Fibrosis Transmembrane Conductance Regulator and Drugs: Insights from Cellular Trafficking

The eukaryotic cell is organized into membrane-delineated compartments that are characterized by specific cadres of proteins sustaining biochemically distinct cellular processes. The appropriate subcellular localization of proteins is key to proper organelle function and provides a physiological context for cellular processes. Disruption of normal trafficking pathways for proteins is seen in several genetic diseases, where a protein's absence for a specific subcellular compartment leads to organelle disruption, and in the context of an individual, a disruption of normal physiology. Importantly, several drug therapies can also alter protein trafficking, causing unwanted side effects. Thus, a deeper understanding of trafficking pathways needs to be appreciated as novel therapeutic modalities are proposed. Despite the promising efficacy of novel therapeutic agents, the intracellular bioavailability of these compounds has proved to be a potential barrier, leading to failures in treatments for various diseases and disorders. While endocytosis of drug moieties provides an efficient means of getting material into cells, the subsequent release and endosomal escape of materials into the cytosol where they need to act has been a barrier. An understanding of cellular protein/lipid trafficking pathways has opened up strategies for increasing drug bioavailability. Approaches to enhance endosomal exit have greatly increased the cytosolic bioavailability of drugs and will provide a means of investigating previous drugs that may have been shelved due to their low cytosolic concentration.

Subcellular Trafficking of the Papillomavirus Genome during Initial Infection: The Remarkable Abilities of Minor Capsid Protein L2

Since 2012, our understanding of human papillomavirus (HPV) subcellular trafficking has undergone a drastic paradigm shift. Work from multiple laboratories has revealed that HPV has evolved a unique means to deliver its viral genome (vDNA) to the cell nucleus, relying on myriad host cell proteins and processes. The major breakthrough finding from these recent endeavors has been the realization of L2-dependent utilization of cellular sorting factors for the retrograde transport of vDNA away from degradative endo/lysosomal compartments to the Golgi, prior to mitosis-dependent nuclear accumulation of L2/vDNA. An overview of current models of HPV entry, subcellular trafficking, and the role of L2 during initial infection is provided below, highlighting unresolved questions and gaps in knowledge.

Protoparvovirus Cell Entry

The Protoparvovirus (PtPV) genus of the Parvoviridae family of viruses includes important animal pathogens and reference molecular models for the entire family. Some virus members of the PtPV genus have arisen as promising tools to treat tumoral processes, as they exhibit marked oncotropism and oncolytic activities while being nonpathogenic for humans. The PtPVs invade and replicate within the nucleus making extensive use of the transport, transcription and replication machineries of the host cells. In order to reach the nucleus, PtPVs need to cross over several intracellular barriers and traffic through different cell compartments, which limit their infection efficiency. In this review we summarize molecular interactions, capsid structural transitions and hijacking of cellular processes, by which the PtPVs enter and deliver their single-stranded DNA genome into the host cell nucleus. Understanding mechanisms that govern the complex PtPV entry will be instrumental in developing approaches to boost their anticancer therapeutic potential and improving their safety profile.

Breach: Host Membrane Penetration and Entry by Nonenveloped Viruses

Disruption of host membranes by nonenveloped viruses, which allows the nucleocapsid or genome to enter the cytosol, is a mechanistically diverse process. Although the membrane-penetrating agents are usually small, hydrophobic or amphipathic peptides deployed from the capsid interior during entry, their manner of membrane interaction varies substantially. In this review, we discuss recent data about the molecular pathways for externalization of viral peptides amidst conformational alterations in the capsid, as well as mechanisms of membrane penetration, which is influenced by structural features of the peptides themselves as well as physicochemical properties of membranes, and other host factors. The membrane-penetrating components of nonenveloped viruses constitute an interesting class of cell-penetrating peptides, and may have potential therapeutic value for gene transfer.

How have retrovirus pseudotypes contributed to our understanding of viral entry?

Study of virus entry into host cells is important for understanding viral tropism and pathogenesis. Studying the entry of in vitro cultured viruses is not always practicable. Study of highly pathogenic viruses, viruses that do not grow in culture, and viruses that rapidly change phenotype in vitro can all benefit from alternative models of entry. Retrovirus particles can be engineered to display the envelope proteins of heterologous enveloped viruses. This approach, broadly termed ‘pseudotyping’, is an important technique for interrogating virus entry. In this perspective we consider how retrovirus pseudotypes have addressed these challenges and improved our understanding of the entry pathways of diverse virus species, including Ebolavirus, human immunodeficiency virus and hepatitis C virus.

Regulated Erlin-dependent release of the B12 transmembrane J-protein promotes ER membrane penetration of a non-enveloped virus

The molecular mechanism by which non-enveloped viruses penetrate biological membranes remains enigmatic. The non-enveloped polyomavirus SV40 penetrates the endoplasmic reticulum (ER) membrane to reach the cytosol and cause infection. We previously demonstrated that SV40 creates its own membrane penetration structure by mobilizing select transmembrane proteins to distinct puncta in the ER membrane called foci that likely function as the cytosol entry sites. How these ER membrane proteins reorganize into the foci is unknown. B12 is a transmembrane J-protein that mobilizes into the foci to promote cytosol entry of SV40. Here we identify two closely related ER membrane proteins Erlin1 and Erlin2 (Erlin1/2) as B12-interaction partners. Strikingly, SV40 recruits B12 to the foci by inducing release of this J-protein from Erlin1/2. Our data thus reveal how a non-enveloped virus promotes its own membrane translocation by triggering the release and recruitment of a critical transport factor to the membrane penetration site.

Polyomavirus (PyV) is a non-enveloped DNA tumor virus that causes debilitating human diseases especially in immunocompromised individuals. At the cellular level, PyVs such as the simian PyV SV40 must enter a host cell and penetrate the ER membrane to reach the cytosol in order to cause infection. Prior to ER membrane transport, SV40 reorganizes select ER membrane proteins including the J-protein B12 to potential membrane penetration sites on the ER membrane called foci where B12 facilitates virus extraction into the cytosol. How B12 reorganizes into the foci is unclear. Here we find that two closely related ER membrane proteins Erlin1 and Erlin2 (Erlin1/2) bind to B12. During infection, SV40 induces release of this J-protein from Erlin1/2 to enable B12 to reorganize into the foci. Our data reveal how a non-enveloped virus mobilizes a specific ER membrane component to a membrane penetration structure to promote its own membrane transport.

SGTA-Dependent Regulation of Hsc70 Promotes Cytosol Entry of Simian Virus 40 from the Endoplasmic Reticulum

Membrane penetration by nonenveloped viruses remains enigmatic. In the case of the nonenveloped polyomavirus simian virus 40 (SV40), the virus penetrates the endoplasmic reticulum (ER) membrane to reach the cytosol and then traffics to the nucleus to cause infection. We previously demonstrated that the cytosolic Hsc70-SGTA-Hsp105 complex is tethered to the ER membrane, where Hsp105 and SGTA facilitate the extraction of SV40 from the ER and transport of the virus into the cytosol. We now find that Hsc70 also ejects SV40 from the ER into the cytosol in a step regulated by SGTA. Although SGTA's N-terminal domain, which mediates homodimerization and recruits cellular adaptors, is dispensable during ER-to-cytosol transport of SV40, this domain appears to exert an unexpected post-ER membrane translocation function during SV40 entry. Our study thus establishes a critical function of Hsc70 within the Hsc70-SGTA-Hsp105 complex in promoting SV40 ER-to-cytosol membrane penetration and unveils a role of SGTA in controlling this step.IMPORTANCE How a nonenveloped virus transports across a biological membrane to cause infection remains mysterious. One enigmatic step is whether host cytosolic components are co-opted to transport the viral particle into the cytosol. During ER-to-cytosol membrane transport of the nonenveloped polyomavirus SV40, a decisive infection step, a cytosolic complex composed of Hsc70-SGTA-Hsp105 was previously shown to associate with the ER membrane. SGTA and Hsp105 have been shown to extract SV40 from the ER and transport the virus into the cytosol. We demonstrate here a critical role of Hsc70 in SV40 ER-to-cytosol penetration and reveal how SGTA controls Hsc70 to impact this process.

Evidence that Hsc70 Is Associated with Cucumber Necrosis Virus Particles and Plays a Role in Particle Disassembly

Uncoating of a virus particle to expose its nucleic acid is a critical aspect of the viral multiplication cycle, as it is essential for the establishment of infection. In the present study, we investigated the role of plant HSP70 homologs in the uncoating process of Cucumber necrosis virus (CNV), a nonenveloped positive-sense single-stranded RNA [(+)ssRNA] virus having a T=3 icosahedral capsid. We have found through Western blot analysis and mass spectrometry that the HSP70 homolog Hsc70-2 copurifies with CNV particles. Virus overlay and immunogold labeling assays suggest that Hsc70-2 is physically bound to virions. Furthermore, trypsin digestion profiles suggest that the bound Hsc70-2 is partially protected by the virus, indicating an intimate association with particles. In investigating a possible role of Hsc70-2 in particle disassembly, we showed that particles incubated with Hsp70/Hsc70 antibody produce fewer local lesions than those incubated with prebleed control antibody on Chenopodium quinoa In conjunction, CNV virions purified using CsCl and having undetectable amounts of Hsc70-2 produce fewer local lesions. We also have found that plants with elevated levels of HSP70/Hsc70 produce higher numbers of local lesions following CNV inoculation. Finally, incubation of recombinant Nicotiana benthamiana Hsc70-2 with virus particles in vitro leads to conformational changes or partial disassembly of capsids as determined by transmission electron microscopy, and particles are more sensitive to chymotrypsin digestion. This is the first report suggesting that a cellular Hsc70 chaperone is involved in disassembly of a plant virus.

IMPORTANCE

Virus particles must disassemble and release their nucleic acid in order to establish infection in a cell. Despite the importance of disassembly in the ability of a virus to infect its host, little is known about this process, especially in the case of nonenveloped spherical RNA viruses. Previous work has shown that host HSP70 homologs play multiple roles in the CNV infection cycle. We therefore examined the potential role of these cellular components in the CNV disassembly process. We show that the HSP70 family member Hsc70-2 is physically associated with CNV virions and that HSP70 antibody reduces the ability of CNV to establish infection. Statistically significantly fewer lesions are produced when virions having undetectable HSc70-2 are used as an inoculum. Finally incubation of Hsc70-2 with CNV particles results in conformational changes in particles. Taken together, our data point to an important role of the host factor Hsc70-2 in CNV disassembly.

[How polyomavirus crosses the endoplasmic reticulum membrane to gain entry into the cytosol]

Polyomavirus (Py) is a non-enveloped, double stranded DNA virus that causes a myriad of devastating human diseases for immunocompromised individuals. To cause infection, Py binds to its receptors on the plasma membrane, is endocytosed, and sorts to the endoplasmic reticulum (ER). From here, Py penetrates the ER membrane to reach the cytosol. Ensuing nuclear entry enables the virus to cause infection. How Py penetrates the ER membrane to access the cytosol is a decisive infection step that is enigmatic. In this review, I highlight the mechanisms by which host cell functions facilitate Py translocation across the ER membrane into the cytosol.

EMC1-dependent stabilization drives membrane penetration of a partially destabilized non-enveloped virus

Destabilization of a non-enveloped virus generates a membrane transport-competent viral particle. Here we probe polyomavirus SV40 endoplasmic reticulum (ER)-to-cytosol membrane transport, a decisive infection step where destabilization initiates this non-enveloped virus for membrane penetration. We find that a member of the ER membrane protein complex (EMC) called EMC1 promotes SV40 ER membrane transport and infection. Surprisingly, EMC1 does so by using its predicted transmembrane residue D961 to bind to and stabilize the membrane-embedded partially destabilized SV40, thereby preventing premature viral disassembly. EMC1-dependent stabilization enables SV40 to engage a cytosolic extraction complex that ejects the virus into the cytosol. Thus EMC1 acts as a molecular chaperone, bracing the destabilized SV40 in a transport-competent state. Our findings reveal the novel principle that coordinated destabilization-stabilization drives membrane transport of a non-enveloped virus.

DOI:http://dx.doi.org/10.7554/eLife.21470.001

Lipids Cooperate with the Reovirus Membrane Penetration Peptide to Facilitate Particle Uncoating

Virus-host interactions play a role in many stages of the viral lifecycle, including entry. Reovirus, a model system for studying the entry mechanisms of nonenveloped viruses, undergoes a series of regulated structural transitions that culminate in delivery of the viral genetic material. Lipids can trigger one of these conformational changes, infectious subviral particle (ISVP)-to-ISVP* conversion. ISVP* formation releases two virally encoded peptides, myristoylated μ1N (myr-μ1N) and Φ. Among these, myr-μ1N is sufficient to form pores within membranes. Released myr-μ1N can also promote ISVP* formation in trans Using thermal inactivation as a readout for ISVP-to-ISVP* conversion, we demonstrate that lipids render ISVPs less thermostable in a virus concentration-dependent manner. Under conditions in which neither lipids alone nor myr-μ1N alone promotes ISVP-to-ISVP* conversion, myr-μ1N induces particle uncoating when lipids are present. These data suggest that the pore-forming activity and the ISVP*-promoting activity of myr-μ1N are linked. Lipid-associated myr-μ1N interacts with ISVPs and triggers efficient ISVP* formation. The cooperativity between a reovirus component and lipids reveals a distinct virus-host interaction in which membranes can facilitate nonenveloped virus entry.

Polyplex micelle installing intracellular self-processing functionalities without free catiomers for safe and efficient systemic gene therapy through tumor vasculature targeting

Both efficiency and safety profiles are crucial for promotion of gene delivery systems towards practical applications. A promising template system was previously developed based on block catiomer of poly(ethylene glycol) (PEG)-b-poly{N'-[N-(2-aminoethyl)-2-aminoehtyl]aspartamide}-cholesteryl [PEG-PAsp(DET)-cholesteryl] with strategies of ligand conjugation at the α-terminus for specific affinity to the targeted cells and cholesteryl conjugation at the ω-terminus for structural stabilization to obtain systemic retention. Aiming for advocating this formulation towards practical applications, in the current study, the binding profile of this polymer to plasmid DNA (pDNA) was carefully studied to address an issue of toxicity origin. Quantification of free polymer composition confirmed that the toxicity mainly results from unbound polymer and polyplex micelle itself has negligible toxicity. This evaluation allowed for identifying an optimal condition to prepare safe polyplex micelles for systemic application that possess maximal polymer-binding but exclude free polymers. The identified polyplex micelles then faced a drawback of limited transfection efficiency due to the absence of free polymer, which is an acknowledged tendency found in various synthetic gene carriers. Thus, series of functional components was strategically compiled to improve the transfection efficiency such as attachment of cyclic (Arg-Gly-Asp) (cRGD) peptide as a ligand onto the polyplex micelles to facilitate cellular uptake, use of endosome membrane disruptive catiomer of PAsp(DET) for facilitating endosome escape along with use of the conjugated cholesteryl group to amplify the effect of PAsp(DET) on membrane disruption, so as to obtain efficient transfection. The mechanistic investigation respecting the appreciated pH dependent protonation behavior of PAsp(DET) permitted to depict an intriguing scenario how the block catiomers manage to escape from the endosome entrapment in response to the pH gradient. Subsequent systemic application to the pancreatic tumor demonstrated a capability of vascular targeting mediated by the cRGD ligand, which was directly confirmed based on in situ confocal laser scanning microscopy observation. Encouraging this result, the vascular targeting to transfect a secretable anti-angiogenic gene was attempted to treat the intractable pancreatic tumor with anticipation that the strategy could circumvent the intrinsic physiological barriers derived from hypovascular and fibrotic characters. The obtained therapeutic efficiency demonstrates promising utilities of the proposed formulation as a safe systemic gene delivery carrier in practical use.

Arms Race between Enveloped Viruses and the Host ERAD Machinery

Enveloped viruses represent a significant category of pathogens that cause serious diseases in animals. These viruses express envelope glycoproteins that are singularly important during the infection of host cells by mediating fusion between the viral envelope and host cell membranes. Despite low homology at protein levels, three classes of viral fusion proteins have, as of yet, been identified based on structural similarities. Their incorporation into viral particles is dependent upon their proper sub-cellular localization after being expressed and folded properly in the endoplasmic reticulum (ER). However, viral protein expression can cause stress in the ER, and host cells respond to alleviate the ER stress in the form of the unfolded protein response (UPR); the effects of which have been observed to potentiate or inhibit viral infection. One important arm of UPR is to elevate the capacity of the ER-associated protein degradation (ERAD) pathway, which is comprised of host quality control machinery that ensures proper protein folding. In this review, we provide relevant details regarding viral envelope glycoproteins, UPR, ERAD, and their interactions in host cells.

Crystal Structures of a Piscine Betanodavirus: Mechanisms of Capsid Assembly and Viral Infection

Betanodaviruses cause massive mortality in marine fish species with viral nervous necrosis. The structure of a T = 3 Grouper nervous necrosis virus-like particle (GNNV-LP) is determined by the ab initio method with non-crystallographic symmetry averaging at 3.6 Å resolution. Each capsid protein (CP) shows three major domains: (i) the N-terminal arm, an inter-subunit extension at the inner surface; (ii) the shell domain (S-domain), a jelly-roll structure; and (iii) the protrusion domain (P-domain) formed by three-fold trimeric protrusions. In addition, we have determined structures of the T = 1 subviral particles (SVPs) of (i) the delta-P-domain mutant (residues 35−217) at 3.1 Å resolution; and (ii) the N-ARM deletion mutant (residues 35−338) at 7 Å resolution; and (iii) the structure of the individual P-domain (residues 214−338) at 1.2 Å resolution. The P-domain reveals a novel DxD motif asymmetrically coordinating two Ca²⁺ ions, and seems to play a prominent role in the calcium-mediated trimerization of the GNNV CPs during the initial capsid assembly process. The flexible N-ARM (N-terminal arginine-rich motif) appears to serve as a molecular switch for T = 1 or T = 3 assembly. Finally, we find that polyethylene glycol, which is incorporated into the P-domain during the crystallization process, enhances GNNV infection. The present structural studies together with the biological assays enhance our understanding of the role of the P-domain of GNNV in the capsid assembly and viral infection by this betanodavirus.

Betanodaviruses belong to the family Nodaviridae and cause the mortality of numerous larval-stage fish species. Here we report protein crystal structures of a piscine betanodavirus, the Grouper nervous necrosis virus (GNNV), in four different forms. Highlights are two structural features that contribute to the viral molecular mechanisms of the T = 3 and T = 1 capsid assembly: a calcium-associated protrusion domain and a functional arginine-rich motif. These results also shed insights into the structural basis for evolutionary lineage of the family Nodaviridae.

Viral Membrane Channels: Role and Function in the Virus Life Cycle

Viroporins are small, hydrophobic trans-membrane viral proteins that oligomerize to form hydrophilic pores in the host cell membranes. These proteins are crucial for the pathogenicity and replication of viruses as they aid in various stages of the viral life cycle, from genome uncoating to viral release. In addition, the ion channel activity of viroporin causes disruption in the cellular ion homeostasis, in particular the calcium ion. Fluctuation in the calcium level triggers the activation of the host defensive programmed cell death pathways as well as the inflammasome, which in turn are being subverted for the viruses’ replication benefits. This review article summarizes recent developments in the functional investigation of viroporins from various viruses and their contributions to viral replication and virulence.

ERdj5 Reductase Cooperates with Protein Disulfide Isomerase To Promote Simian Virus 40 Endoplasmic Reticulum Membrane Translocation

The nonenveloped polyomavirus (PyV) simian virus 40 (SV40) traffics from the cell surface to the endoplasmic reticulum (ER), where it penetrates the ER membrane to reach the cytosol before mobilizing into the nucleus to cause infection. Prior to ER membrane penetration, ER lumenal factors impart structural rearrangements to the virus, generating a translocation-competent virion capable of crossing the ER membrane. Here we identify ERdj5 as an ER enzyme that reduces SV40's disulfide bonds, a reaction important for its ER membrane transport and infection. ERdj5 also mediates human BK PyV infection. This enzyme cooperates with protein disulfide isomerase (PDI), a redox chaperone previously implicated in the unfolding of SV40, to fully stimulate membrane penetration. Negative-stain electron microscopy of ER-localized SV40 suggests that ERdj5 and PDI impart structural rearrangements to the virus. These conformational changes enable SV40 to engage BAP31, an ER membrane protein essential for supporting membrane penetration of the virus. Uncoupling of SV40 from BAP31 traps the virus in ER subdomains called foci, which likely serve as depots from where SV40 gains access to the cytosol. Our study thus pinpoints two ER lumenal factors that coordinately prime SV40 for ER membrane translocation and establishes a functional connection between lumenal and membrane events driving this process.

IMPORTANCE

PyVs are established etiologic agents of many debilitating human diseases, especially in immunocompromised individuals. To infect cells at the cellular level, this virus family must penetrate the host ER membrane to reach the cytosol, a critical entry step. In this report, we identify two ER lumenal factors that prepare the virus for ER membrane translocation and connect these lumenal events with events on the ER membrane. Pinpointing cellular components necessary for supporting PyV infection should lead to rational therapeutic strategies for preventing and treating PyV-related diseases.

A nucleotide exchange factor promotes endoplasmic reticulum-to-cytosol membrane penetration of the nonenveloped virus simian virus 40

The nonenveloped simian polyomavirus (PyV) simian virus 40 (SV40) hijacks the endoplasmic reticulum (ER) quality control machinery to penetrate the ER membrane and reach the cytosol, a critical infection step. During entry, SV40 traffics to the ER, where host-induced conformational changes render the virus hydrophobic. The hydrophobic virus binds and integrates into the ER lipid bilayer to initiate membrane penetration. However, prior to membrane transport, the hydrophobic SV40 recruits the ER-resident Hsp70 BiP, which holds the virus in a transport-competent state until it is ready to cross the ER membrane. Here we probed how BiP disengages from SV40 to enable the virus to penetrate the ER membrane. We found that nucleotide exchange factor (NEF) Grp170 induces nucleotide exchange of BiP and releases SV40 from BiP. Importantly, this reaction promotes SV40 ER-to-cytosol transport and infection. The human BK PyV also relies on Grp170 for successful infection. Interestingly, SV40 mobilizes a pool of Grp170 into discrete puncta in the ER called foci. These foci, postulated to represent the ER membrane penetration site, harbor ER components, including BiP, known to facilitate viral ER-to-cytosol transport. Our results thus identify a nucleotide exchange activity essential for catalyzing the most proximal event before ER membrane penetration of PyVs.

IMPORTANCE

PyVs are known to cause debilitating human diseases. During entry, this virus family, including monkey SV40 and human BK PyV, hijacks ER protein quality control machinery to breach the ER membrane and access the cytosol, a decisive infection step. In this study, we pinpointed an ER-resident factor that executes a crucial role in promoting ER-to-cytosol membrane penetration of PyVs. Identifying a host factor that facilitates entry of the PyV family thus provides additional therapeutic targets to combat PyV-induced diseases.

Getting across the cell membrane: an overview for small molecules, peptides, and proteins

The ability to efficiently access cytosolic proteins is desired in both biological research and medicine. However, targeting intracellular proteins is often challenging, because to reach the cytosol, exogenous molecules must first traverse the cell membrane. This review provides a broad overview of how certain molecules are thought to cross this barrier, and what kinds of approaches are being made to enhance the intracellular delivery of those that are impermeable. We first discuss rules that govern the passive permeability of small molecules across the lipid membrane, and mechanisms of membrane transport that have evolved in nature for certain metabolites, peptides, and proteins. Then, we introduce design strategies that have emerged in the development of small molecules and peptides with improved permeability. Finally, intracellular delivery systems that have been engineered for protein payloads are surveyed. Viewpoints from varying disciplines have been brought together to provide a cohesive overview of how the membrane barrier is being overcome.

Nonenveloped capsid: mechanisms of viral entry

ABSTRACT 

Viruses are a diverse class of nanoparticles. However, they have evolved a few common mechanisms that enable successful infection of their host cells. The first stage of this process involves entry into the cell. For enveloped viruses this process has been well characterized. For nonenveloped viruses, the focus of this review, the entry mechanisms are less well understood. For these viruses, a typical pathway involves receptor attachment followed by internalization into cellular vesicles and subsequent viral escape to the cytosol and transport to the site of genome replication. Significantly, these viruses have evolved numerous mechanisms to fulfill this seemingly simple infection scheme. We focus on the latest observations for several families of nonenveloped viruses and highlight specific members for eukaryotic families: Adenoviridae, Papillomaviridae, Parvoviridae, Picornaviridae, Polyomaviridae and Reoviridae; and prokaryotic families: Microviridae, Myoviridae, Podoviridae and Siphoviridae.

Rotavirus entry: a deep journey into the cell with several exits

Rotaviruses are the leading etiological agents of acute gastroenteritis in infants and young children worldwide. These nonenveloped viruses enter cells using different types of endocytosis and, depending on the virus strain, travel to different endosomal compartments before exiting to the cytosolic space. In this Gem, we review the viral and cellular factors involved in the different stages of a productive virus cell entry and share with the readers the journey that we have taken into the cell to learn about virus entry.

Development and Challenges of Nanovectors in Gene Therapy

In recent years, hundreds of genes have been linked to a variety of human diseases, and the field of gene therapy has emerged as a way to treat this wide range of diseases. The main goal of gene therapy is to find a gene delivery vehicle that can successfully target diseased cells and deliver therapeutic genes directly to their cellular compartment. The two main types of gene delivery vectors currently being investigated in clinical trials are recombinant viral vectors and synthetic nonviral vectors. Recombinant viral vectors take advantage of the evolutionarily optimized viral mechanisms to deliver genes, but they can be hard to specifically target in vivo and are also associated with serious side effects. Synthetic nonviral vectors are made out of highly biocompatible lipids or polymers, but they are much less efficient at delivering their genetic payload due to the lack of any active delivery mechanism. This mini review will introduce the current state of gene delivery in clinical trials, and discuss the specific challenges associated with each of these vectors. It will also highlight some specific gaps in knowledge that are limiting the advancement of this field and touch on the current areas of research being explored to overcome them.

The VP4 peptide of hepatitis A virus ruptures membranes through formation of discrete pores

Membrane-active peptides, components of capsid structural proteins, assist viruses in overcoming the host membrane barrier in the initial stages of infection. Several such peptides have been identified, and their roles in membrane fusion or disruption have been characterized through biophysical studies. In several members of the Picornaviridae family, the role of the VP4 structural peptide in cellular-membrane penetration is well established. However, there is not much information on the membrane-penetrating capsid components of hepatitis A virus (HAV), an unusual member of this family. The VP4 peptide of HAV differs from its analogues in other picornaviruses in being significantly shorter in length and in lacking a signal for myristoylation, thought to be a critical requisite for VP4-mediated membrane penetration. Here we report, for the first time, that the atypical VP4 in HAV contains significant membrane-penetrating activity. Using a combination of biophysical assays and molecular dynamics simulation studies, we show that VP4 integrates into membrane vesicles through its N-terminal region to finally form discrete pores of 5- to 9-nm diameter, which induces leakage in the vesicles without altering their overall size or shape. We further demonstrate that the membrane activity of VP4 is specific toward vesicles mimicking the lipid content of late endosomes at acidic pH. Taken together, our data indicate that VP4 might be essential for the penetration of host endosomal membranes and release of the viral genome during HAV entry.

IMPORTANCE

Hepatitis A virus causes acute hepatitis in humans through the fecal-oral route and is particularly prevalent in underdeveloped regions with poor hygienic conditions. Although a vaccine for HAV exists, its high cost makes it unsuitable for universal application in developing countries. Studies on host-virus interaction for HAV have been hampered due to a lack of starting material, since the virus is extremely slow growing in culture. Among the unknown aspects of the HAV life cycle is its manner of host membrane penetration, which is one of the most important initial steps in viral infection. Here, we present data to suggest that a small peptide, VP4, a component of the HAV structural polyprotein, might be essential in helping the viral genome cross cell membranes during entry. It is hoped that this work might help in elucidating the manner of initial host cell interaction by HAV.

A cytosolic chaperone complexes with dynamic membrane J-proteins and mobilizes a nonenveloped virus out of the endoplasmic reticulum

Nonenveloped viruses undergo conformational changes that enable them to bind to, disrupt, and penetrate a biological membrane leading to successful infection. We assessed whether cytosolic factors play any role in the endoplasmic reticulum (ER) membrane penetration of the nonenveloped SV40. We find the cytosolic SGTA-Hsc70 complex interacts with the ER transmembrane J-proteins DnaJB14 (B14) and DnaJB12 (B12), two cellular factors previously implicated in SV40 infection. SGTA binds directly to SV40 and completes ER membrane penetration. During ER-to-cytosol transport of SV40, SGTA disengages from B14 and B12. Concomitant with this, SV40 triggers B14 and B12 to reorganize into discrete foci within the ER membrane. B14 must retain its ability to form foci and interact with SGTA-Hsc70 to promote SV40 infection. Our results identify a novel role for a cytosolic chaperone in the membrane penetration of a nonenveloped virus and raise the possibility that the SV40-induced foci represent cytosol entry sites.