A large and intact viral particle penetrates the endoplasmic reticulum membrane to reach the cytosol

Mouse polyomavirus infection induces lamin reorganisation

The nuclear lamina is a dense network of intermediate filaments beneath the inner nuclear membrane. Composed of A-type lamins (lamin A/C) and B-type lamins (lamins B1 and B2), the nuclear lamina provides a scaffold for the nuclear envelope and chromatin, thereby maintaining the structural integrity of the nucleus. A-type lamins are also found inside the nucleus where they interact with chromatin and participate in gene regulation. Viruses replicating in the cell nucleus have to overcome the nuclear envelope during the initial phase of infection and during the nuclear egress of viral progeny. Here, we focused on the role of lamins in the replication cycle of a dsDNA virus, mouse polyomavirus. We detected accumulation of the major capsid protein VP1 at the nuclear periphery, defects in nuclear lamina staining and different lamin A/C phosphorylation patterns in the late phase of mouse polyomavirus infection, but the nuclear envelope remained intact. An absence of lamin A/C did not affect the formation of replication complexes but did slow virus propagation. Based on our findings, we propose that the nuclear lamina is a scaffold for replication complex formation and that lamin A/C has a crucial role in the early phases of infection with mouse polyomavirus.

Polyomavirus Wakes Up and Chooses Neurovirulence

JC polyomavirus (JCPyV) is a human-specific polyomavirus that establishes a silent lifelong infection in multiple peripheral organs, predominantly those of the urinary tract, of immunocompetent individuals. In immunocompromised settings, however, JCPyV can infiltrate the central nervous system (CNS), where it causes several encephalopathies of high morbidity and mortality. JCPyV-induced progressive multifocal leukoencephalopathy (PML), a devastating demyelinating brain disease, was an AIDS-defining illness before antiretroviral therapy that has "reemerged" as a complication of immunomodulating and chemotherapeutic agents. No effective anti-polyomavirus therapeutics are currently available. How depressed immune status sets the stage for JCPyV resurgence in the urinary tract, how the virus evades pre-existing antiviral antibodies to become viremic, and where/how it enters the CNS are incompletely understood. Addressing these questions requires a tractable animal model of JCPyV CNS infection. Although no animal model can replicate all aspects of any human disease, mouse polyomavirus (MuPyV) in mice and JCPyV in humans share key features of peripheral and CNS infection and antiviral immunity. In this review, we discuss the evidence suggesting how JCPyV migrates from the periphery to the CNS, innate and adaptive immune responses to polyomavirus infection, and how the MuPyV-mouse model provides insights into the pathogenesis of JCPyV CNS disease.

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.

Components of the LINC and NPC complexes coordinately target and translocate a virus into the nucleus to promote infection

Nuclear entry represents the final and decisive infection step for most DNA viruses, although how this is accomplished by some viruses is unclear. Polyomavirus SV40 transports from the cell surface through the endosome, the endoplasmic reticulum, and the cytosol from where it enters the nucleus to cause infection. Here we elucidate the nuclear entry mechanism of SV40. Our results show that cytosol-localized SV40 is targeted to the nuclear envelope by directly engaging Nesprin-2 of the linker of nucleoskeleton and cytoskeleton (LINC) nuclear membrane complex. Additionally, we identify the NUP188 subunit of the nuclear pore complex (NPC) as a new Nesprin-2-interacting partner. This physical proximity positions the NPC to capture SV40 upon release from Nesprin-2, enabling the channel to facilitate nuclear translocation of the virus. Strikingly, SV40 disassembles during nuclear entry, generating a viral genome-VP1-VP3 subcomplex that efficiently crosses the NPC to enter the nucleus. Our results reveal how two major nuclear membrane protein complexes are exploited to promote targeting and translocation of a virus into the nucleus.

Packaging of DNA origami in viral capsids: towards synthetic viruses

We report a new type of nanoparticle, consisting of a nucleic acid core (>7500 nt) folded into a 35 nm DNA origami sphere, encapsulated by a capsid composed of all three SV40 virus capsid proteins. Compared to the prototype reported previously, whose capsid consists of VP1 only, the new nanoparticle closely adopts the unique intracellular pathway of the native SV40, suggesting that the proteins of the synthetic capsid retain their native viral functionality. Some of the challenges in the design of such near-future composite drugs destined for gene delivery are discussed.

High-Throughput Screening of FDA-Approved Drug Library Reveals Ixazomib Is a Broad-Spectrum Antiviral Agent against Arboviruses

The emergence of significant arboviruses and their spillover transmission to humans represent a major threat to global public health. No approved drugs are available for the treatment of significant arboviruses in circulation today. The repurposing of clinically approved drugs is one of the most rapid and promising strategies in the identification of effective treatments for diseases caused by arboviruses. Here, we screened small-molecule compounds with anti-tick-borne encephalitis virus, West Nile virus, yellow fever virus and chikungunya virus activity from 2580 FDA-approved drugs. In total, 60 compounds showed antiviral efficacy against all four of the arboviruses in Huh7 cells. Among these compounds, ixazomib and ixazomib citrate (inhibitors of 20S proteasome β5) exerted antiviral effects at a low-micromolar concentration. The time-of-drug-addition assay suggested that ixazomib and ixazomib citrate disturbed multiple processes in viruses’ life cycles. Furthermore, ixazomib and ixazomib citrate potently inhibited chikungunya virus replication and relieved virus-induced footpad swelling in a mouse model. These results offer critical information which supports the role of ixazomib as a broad-spectrum agent against arboviruses.

Membrane Translocation of Folded Proteins

An ever-increasing number of proteins have been shown to translocate across various membranes of bacterial as well as eukaryotic cells in their folded states as a part of physiological and/or pathophysiological processes. Herein we provide an overview of the systems/processes that are established or likely to involve the membrane translocation of folded proteins, such as protein export by the twin-arginine translocation (TAT) system in bacteria and chloroplasts, unconventional protein secretion (UPS) and protein import into the peroxisome in eukaryotes, and the cytosolic entry of proteins (e.g., bacterial toxins) and viruses into eukaryotes. We also discuss the various mechanistic models that have previously been proposed for the membrane translocation of folded proteins including pore/channel formation, local membrane disruption, membrane thinning, and transport by membrane vesicles. Finally, we introduce a newly discovered vesicular transport mechanism, vesicle budding and collapse (VBC), and present evidence that VBC may represent a unifying mechanism that drives some (and potentially all) of folded protein translocation processes.

Investigation of the Crosstalk between GRP78/PERK/ATF-4 Signaling Pathway and Renal Apoptosis Induced by Nephropathogenic Infectious Bronchitis Virus Infection

This study aims to explore the crosstalk between GRP78/PERK/ATF-4 signaling pathway and renal apoptosis induced by nephropathogenic infectious bronchitis virus (NIBV). Hy-Line brown chickens were divided into two groups (Con, n = 100 and Dis, n = 200). At 28 days of age, each chicken in the Dis group was intranasally injected with SX9 strain (10-5/0.2 ml). Venous blood and kidney tissues were collected at 1, 5, 11, 18 and 28 days postinfection. Our results showed that NIBV infection upregulated the levels of creatinine, uric acid, and calcium (Ca2+) levels. Histopathological examination revealed severe hemorrhage and inflammatory cell infiltration near the renal tubules. Meanwhile, NIBV virus particles and apoptotic bodies were observed by ultramicro electron microscope. In addition, RT-qPCR and Western blot showed that NIBV upregulated the expression of GRP78, PERK, eIF2α, ATF-4, CHOP, Caspase-3, Caspase-9, P53, Bax, and on the contrary, downregulated the expression of Bcl-2. Furthermore, immunofluorescence localization analysis showed that the positive expression of Bcl-2 protein was significantly decreased. Correlation analysis indicated that endoplasmic reticulum (ER) stress gene expression, apoptosis gene expression, and renal injury were potentially related. Taken together, NIBV infection can induce renal ER stress and apoptosis by activating of GRP78/PERK/ATF-4 signaling pathway, leading to kidney damage. IMPORTANCE Nephropathogenic infectious bronchitis virus (NIBV) induced renal endoplasmic reticulum stress in chickens. NIBV infection induced kidney apoptosis in chickens. GRP78/PERK/ATF-4 signaling pathway is potentially related to renal apoptosis induced by NIBV.

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.

Immune sensing of mouse polyomavirus DNA by p204 and cGAS DNA sensors

The mechanism by which DNA viruses interact with different DNA sensors and their connection with the activation of interferon (IFN) type I pathway are poorly understood. We investigated the roles of protein 204 (p204) and cyclic guanosine-adenosine synthetase (cGAS) sensors during infection with mouse polyomavirus (MPyV). The phosphorylation of IFN regulatory factor 3 (IRF3) and the stimulator of IFN genes (STING) proteins and the upregulation of IFN beta (IFN-β) and MX Dynamin Like GTPase 1 (MX-1) genes were detected at the time of replication of MPyV genomes in the nucleus. STING knockout abolished the IFN response. Infection with a mutant virus that exhibits defective nuclear entry via nucleopores and that accumulates in the cytoplasm confirmed that replication of viral genomes in the nucleus is required for IFN induction. The importance of both DNA sensors, p204 and cGAS, in MPyV-induced IFN response was demonstrated by downregulation of the IFN pathway observed in p204-knockdown and cGAS-knockout cells. Confocal microscopy revealed the colocalization of p204 with MPyV genomes in the nucleus. cGAS was found in the cytoplasm, colocalizing with viral DNA leaked from the nucleus and with DNA within micronucleus-like bodies, but also with the MPyV genomes in the nucleus. However, 2'3'-Cyclic guanosine monophosphate-adenosine monophosphate synthesized by cGAS was detected exclusively in the cytoplasm. Biochemical assays revealed no evidence of functional interaction between cGAS and p204 in the nucleus. Our results provide evidence for the complex interactions of MPyV and DNA sensors including the sensing of viral genomes in the nucleus by p204 and of leaked viral DNA and micronucleus-like bodies in the cytoplasm by cGAS.

Manipulation of Host Cell Organelles by Intracellular Pathogens

Pathogenic intracellular bacteria, parasites and viruses have evolved sophisticated mechanisms to manipulate mammalian host cells to serve as niches for persistence and proliferation. The intracellular lifestyles of pathogens involve the manipulation of membrane-bound organellar compartments of host cells. In this review, we described how normal structural organization and cellular functions of endosomes, endoplasmic reticulum, Golgi apparatus, mitochondria, or lipid droplets are targeted by microbial virulence mechanisms. We focus on the specific interactions of Salmonella, Legionella pneumophila, Rickettsia rickettsii, Chlamydia spp. and Mycobacterium tuberculosis representing intracellular bacterial pathogens, and of Plasmodium spp. and Toxoplasma gondii representing intracellular parasites. The replication strategies of various viruses, i.e., Influenza A virus, Poliovirus, Brome mosaic virus, Epstein-Barr Virus, Hepatitis C virus, severe acute respiratory syndrome virus (SARS), Dengue virus, Zika virus, and others are presented with focus on the specific manipulation of the organelle compartments. We compare the specific features of intracellular lifestyle and replication cycles, and highlight the communalities in mechanisms of manipulation deployed.

How DNA and RNA Viruses Exploit Host Chaperones to Promote Infection

To initiate infection, a virus enters a host cell typically via receptor-dependent endocytosis. It then penetrates a subcellular membrane, reaching a destination that supports transcription, translation, and replication of the viral genome. These steps lead to assembly and morphogenesis of the new viral progeny. The mature virus finally exits the host cell to begin the next infection cycle. Strikingly, viruses hijack host molecular chaperones to accomplish these distinct entry steps. Here we highlight how DNA viruses, including polyomavirus and the human papillomavirus, exploit soluble and membrane-associated chaperones to enter a cell, penetrating and escaping an intracellular membrane en route for infection. We also describe the mechanism by which RNA viruses—including flavivirus and coronavirus—co-opt cytosolic and organelle-selective chaperones to promote viral endocytosis, protein biosynthesis, replication, and assembly. These examples underscore the importance of host chaperones during virus infection, potentially revealing novel antiviral strategies to combat virus-induced diseases.

GRK2 mediates β-arrestin interactions with 5-HT2 receptors for JC polyomavirus endocytosis

JC polyomavirus (JCPyV) infects the majority of the population, establishing a lifelong, asymptomatic infection in the kidney of healthy individuals. People that become severely immunocompromised may experience JCPyV reactivation, which can cause progressive multifocal leukoencephalopathy (PML), a neurodegenerative disease. Due to a lack of therapeutic options, PML results in fatality or significant debilitation among affected individuals. Cellular internalization of JCPyV is mediated by serotonin 5-hydroxytryptamine subfamily 2 receptors (5-HT₂Rs) via clathrin-mediated endocytosis. The JCPyV entry process requires the clathrin-scaffolding proteins β-arrestin, adaptor protein 2 (AP2), and dynamin. Further, a β-arrestin interacting domain, the Ala-Ser-Lys (ASK) motif, within the C-terminus of 5-HT_2AR is important for JCPyV internalization and infection. Interestingly, 5-HT₂R subtypes A, B, and C equally support JCPyV entry and infection, and all subtypes contain an ASK motif, suggesting a conserved mechanism for viral entry. However, the role of the 5-HT₂R ASK motifs and the activation of β-arrestin-associated proteins during internalization has not been fully elucidated. Through mutagenesis, the ASK motifs within 5-HT_2BR and 5-HT_2CR were identified as critical for JCPyV internalization and infectivity. Further, utilizing biochemical pulldown techniques, mutagenesis of the ASK motifs in 5-HT_2BR and 5-HT_2CR resulted in reduced β-arrestin binding. Utilizing small-molecule chemical inhibitors and RNA interference, G-protein receptor kinase 2 (GRK2) was determined to be required for JCPyV internalization and infection by mediating interactions between β-arrestin and the ASK motif of 5-HT₂Rs. These findings demonstrate that GRK2 and β-arrestin interactions with 5-HT₂Rs are critical for JCPyV entry by clathrin-mediated endocytosis and resultant infection.IMPORTANCE As intracellular parasites, viruses require a host cell to replicate and cause disease. Therefore, virus-host interactions contribute to viral pathogenesis. JC polyomavirus (JCPyV) infects most of the population, establishing a lifelong asymptomatic infection within the kidney. Under conditions of severe immunosuppression JCPyV may spread to the central nervous system, causing the fatal demyelinating disease progressive multifocal leukoencephalopathy (PML). Individuals living with HIV or undergoing immunomodulatory therapies are at risk for developing PML. The mechanisms of how JCPyV uses specific receptors on the surface of host cells to initiate internalization and infection is a poorly understood process. We have further identified cellular proteins involved in JCPyV internalization and infection and elucidated their specific interactions that are responsible for activation of receptors. Collectively, these findings illuminate how viruses usurp cellular receptors during infection, contributing to current development efforts for therapeutic options for the treatment or prevention of PML.

Sending mixed signals: polyomavirus entry and trafficking

Polyomaviruses are mostly non-pathogenic, yet some can cause human disease especially under conditions of immunosuppression, including JC, BK, and Merkel cell polyomaviruses. Direct interactions between viruses and the host early during infection dictate the outcome of disease, many of which remain enigmatic. However, significant work in recent years has contributed to our understanding of how this virus family establishes an infection, largely due to advances made for animal polyomaviruses murine and SV40. Here we summarize the major findings that have contributed to our understanding of polyomavirus entry, trafficking, disassembly, signaling, and immune evasion during the infectious process and highlight major unknowns in these processes that are open areas of study.

Golgi-associated BICD adaptors couple ER membrane penetration and disassembly of a viral cargo

During entry, viruses must navigate through the host endomembrane system, penetrate cellular membranes, and undergo capsid disassembly to reach an intracellular destination that supports infection. How these events are coordinated is unclear. Here, we reveal an unexpected function of a cellular motor adaptor that coordinates virus membrane penetration and disassembly. Polyomavirus SV40 traffics to the endoplasmic reticulum (ER) and penetrates a virus-induced structure in the ER membrane called “focus” to reach the cytosol, where it disassembles before nuclear entry to promote infection. We now demonstrate that the ER focus is constructed proximal to the Golgi-associated BICD2 and BICDR1 dynein motor adaptors; this juxtaposition enables the adaptors to directly bind to and disassemble SV40 upon arrival to the cytosol. Our findings demonstrate that positioning of the virus membrane penetration site couples two decisive infection events, cytosol arrival and disassembly, and suggest cargo remodeling as a novel function of dynein adaptors.

ER functions are exploited by viruses to support distinct stages of their life cycle

The endoplasmic reticulum (ER), with its expansive membranous system and a vast network of chaperones, enzymes, sensors, and ion channels, orchestrates diverse cellular functions, ranging from protein synthesis, folding, secretion, and degradation to lipid biogenesis and calcium homeostasis. Strikingly, some of the functions of the ER are exploited by viruses to promote their life cycles. During entry, viruses must penetrate a host membrane and reach an intracellular destination to express and replicate their genomes. These events lead to the assembly of new viral progenies that exit the host cell, thereby initiating further rounds of infection. In this review, we highlight how three distinct viruses - polyomavirus, flavivirus, and coronavirus - co-opt key functions of the ER to cause infection. We anticipate that illuminating this virus-ER interplay will provide rational therapeutic approaches to combat the virus-induced diseases.

Viral entry and the ubiquitin-proteasome system

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

Taking the Scenic Route: Polyomaviruses Utilize Multiple Pathways to Reach the Same Destination

Members of the Polyomaviridae family differ in their host range, pathogenesis, and disease severity. To date, some of the most studied polyomaviruses include human JC, BK, and Merkel cell polyomavirus and non-human subspecies murine and simian virus 40 (SV40) polyomavirus. Although dichotomies in host range and pathogenesis exist, overlapping features of the infectious cycle illuminate the similarities within this virus family. Of particular interest to human health, JC, BK, and Merkel cell polyomavirus have all been linked to critical, often fatal, illnesses, emphasizing the importance of understanding the underlying viral infections that result in the onset of these diseases. As there are significant overlaps in the capacity of polyomaviruses to cause disease in their respective hosts, recent advancements in characterizing the infectious life cycle of non-human murine and SV40 polyomaviruses are key to understanding diseases caused by their human counterparts. This review focuses on the molecular mechanisms by which different polyomaviruses hijack cellular processes to attach to host cells, internalize, traffic within the cytoplasm, and disassemble within the endoplasmic reticulum (ER), prior to delivery to the nucleus for viral replication. Unraveling the fundamental processes that facilitate polyomavirus infection provides deeper insight into the conserved mechanisms of the infectious process shared within this virus family, while also highlighting critical unique viral features.

JCPyV VP1 Mutations in Progressive MultifocalLeukoencephalopathy: Altering Tropismor Mediating Immune Evasion?

Polyomaviruses are ubiquitous human pathogens that cause lifelong, asymptomatic infections in healthy individuals. Although these viruses are restrained by an intact immune system, immunocompromised individuals are at risk for developing severe diseases driven by resurgent viral replication. In particular, loss of immune control over JC polyomavirus can lead to the development of the demyelinating brain disease progressive multifocal leukoencephalopathy (PML). Viral isolates from PML patients frequently carry point mutations in the major capsid protein, VP1, which mediates virion binding to cellular glycan receptors. Because polyomaviruses are non-enveloped, VP1 is also the target of the host's neutralizing antibody response. Thus, VP1 mutations could affect tropism and/or recognition by polyomavirus-specific antibodies. How these mutations predispose susceptible individuals to PML and other JCPyV-associated CNS diseases remains to be fully elucidated. Here, we review the current understanding of polyomavirus capsid mutations and their effects on viral tropism, immune evasion, and virulence.

Reticulon protects the integrity of the ER membrane during ER escape of large macromolecular protein complexes

Escape of large macromolecular complexes from the endoplasmic reticulum (ER), such as a viral particle or cellular aggregate, likely induces mechanical stress initiated on the luminal side of the ER membrane, which may threaten its integrity. How the ER responds to this threat remains unknown. Here we demonstrate that the cytosolic leaflet ER morphogenic protein reticulon (RTN) protects ER membrane integrity when polyomavirus SV40 escapes the ER to reach the cytosol en route to infection. SV40 coopts an intrinsic RTN function, as we also found that RTN prevents membrane damage during ER escape of a misfolded proinsulin aggregate destined for lysosomal degradation via ER-phagy. Our studies reveal that although ER membrane integrity may be threatened during ER escape of large macromolecular protein complexes, the action of RTN counters this, presumably by deploying its curvature-inducing activity to provide membrane flexibility and stability to limit mechanical stress imposed on the ER membrane.

"Repair Me if You Can": Membrane Damage, Response, and Control from the Viral Perspective

Cells are constantly challenged by pathogens (bacteria, virus, and fungi), and protein aggregates or chemicals, which can provoke membrane damage at the plasma membrane or within the endo-lysosomal compartments. Detection of endo-lysosomal rupture depends on a family of sugar-binding lectins, known as galectins, which sense the abnormal exposure of glycans to the cytoplasm upon membrane damage. Galectins in conjunction with other factors orchestrate specific membrane damage responses such as the recruitment of the endosomal sorting complex required for transport (ESCRT) machinery to either repair damaged membranes or the activation of autophagy to remove membrane remnants. If not controlled, membrane damage causes the release of harmful components including protons, reactive oxygen species, or cathepsins that will elicit inflammation. In this review, we provide an overview of current knowledge on membrane damage and cellular responses. In particular, we focus on the endo-lysosomal damage triggered by non-enveloped viruses (such as adenovirus) and discuss viral strategies to control the cellular membrane damage response. Finally, we debate the link between autophagy and inflammation in this context and discuss the possibility that virus induced autophagy upon entry limits inflammation.

Ubqln4 Facilitates Endoplasmic Reticulum-to-Cytosol Escape of a Nonenveloped Virus during Infection

The nonenveloped polyomavirus simian virus 40 (SV40) must penetrate the host endoplasmic reticulum (ER) membrane to enter the cytosol in order to promote infection. How this is accomplished is not entirely clear. Here, we demonstrate that the cytosolic chaperone Ubiquilin4 (Ubqln4) binds directly to the ER membrane J proteins B12 and B14. Strategically localized at the ER-cytosol interface, Ubqln4 captures SV40 emerging from the ER, thereby facilitating escape of the virus from the ER into the cytosol, which leads to infection. Strikingly, Ubqln4 engages the J proteins in a J-domain-independent manner, in contrast to the previously reported Hsc70-Hsp105-SGTA-Bag2 cytosolic complex that also mediates SV40 ER-to-cytosol transport. Our results also reveal that the H domain and STI1 motif (1-2) of Ubqln4 support J protein binding, essential for SV40 infection. Together, these data further clarify the molecular basis by which a nonenveloped virus escapes a host membrane during infectious entry.IMPORTANCE How a nonenveloped virus escapes from a host membrane to promote infection remains enigmatic. In the case of the nonenveloped polyomavirus SV40, penetration of the ER membrane to reach the cytosol is a decisive virus infection step. In this study, we found a new host factor called Ubqln4 that facilitates escape of SV40 from the ER into the cytosol, thereby providing a path for the virus to enter the nucleus to cause infection.

Selective EMC subunits act as molecular tethers of intracellular organelles exploited during viral entry

Although viruses must navigate the complex host endomembrane system to infect cells, the strategies used to achieve this is unclear. During entry, polyomavirus SV40 is sorted from the late endosome (LE) to the endoplasmic reticulum (ER) to cause infection, yet how this is accomplished remains enigmatic. Here we find that EMC4 and EMC7, two ER membrane protein complex (EMC) subunits, support SV40 infection by promoting LE-to-ER targeting of the virus. They do this by engaging LE-associated Rab7, presumably to stabilize contact between the LE and ER. These EMC subunits also bind to the ER-resident fusion machinery component syntaxin18, which is required for SV40-arrival to the ER. Our data suggest that EMC4 and EMC7 act as molecular tethers, inter-connecting two intracellular compartments to enable efficient transport of a virus between these compartments. As LE-to-ER transport of cellular cargos is unclear, our results have broad implications for illuminating inter-organelle cargo transport.

The endoplasmic reticulum membrane protein complex (EMC) is known to play a role in SV40 viral infection but precise mechanisms are unclear. Here, the authors report that the EMC acts as tether of late endosome–endoplasmic reticulum interorganellar membrane contact sites to promote SV40 viral infection.

Influenza A virus uncoating

Influenza A virus (IAV) is an enveloped virus of the Orthomyxoviridae with a negative-sense single-stranded RNA genome. During virus cell entry, viral and cellular cues are delivered in a stepwise manner within two distinct cellular compartments-the endosomes and the cytosol. Endosome maturation primes the viral core for uncoating by cytosolic host proteins and host-mediated virus disaggregation is essential for genome import and replication in the nucleus. Recent evidence shows that two well-known cellular proteins-histone deacetylase 6 (HDAC6) and karyopherin-β2 (kapβ2)-uncoat influenza virus. HDAC6 is 1 of 11 HDACs and an X-linked, cytosolic lysine deacetylase. Under normal cellular conditions HDAC6 is the tubulin deacetylase. Under proteasomal stress HDAC6 binds unanchored ubiquitin, dynein and myosin II to sequester misfolded protein aggregates for autophagy. Kapβ2 is a member of the importin β family that transports RNA-binding proteins into the nucleus by binding to disordered nuclear localization signals (NLSs) known as PY-NLS. Kapβ2 is emerging as a universal uncoating factor for IAV and human immunodeficiency virus type 1 (HIV-1). Kapβ2 can also reverse liquid-liquid phase separation (LLPS) of RNA-binding proteins by promoting their disaggregation. Thus, it is becoming evident that key players in the management of cellular condensates and membraneless organelles are potent virus uncoating factors. This emerging concept reveals implications in viral pathogenesis, as well as, the promise for cell-targeted therapeutic strategies to block universal virus uncoating pathways hijacked by enveloped RNA viruses.

A polyomavirus peptide binds to the capsid VP1 pore and has potent antiviral activity against BK and JC polyomaviruses

In pursuit of therapeutics for human polyomaviruses, we identified a peptide derived from the BK polyomavirus (BKV) minor structural proteins VP2/3 that is a potent inhibitor of BKV infection with no observable cellular toxicity. The thirteen-residue peptide binds to major structural protein VP1 with single-digit nanomolar affinity. Alanine-scanning of the peptide identified three key residues, substitution of each of which results in ~1000 fold loss of binding affinity with a concomitant reduction in antiviral activity. Structural studies demonstrate specific binding of the peptide to the pore of pentameric VP1. Cell-based assays demonstrate nanomolar inhibition (EC₅₀) of BKV infection and suggest that the peptide acts early in the viral entry pathway. Homologous peptide exhibits similar binding to JC polyomavirus VP1 and inhibits infection with similar potency to BKV in a model cell line. Lastly, these studies validate targeting the VP1 pore as a novel strategy for the development of anti-polyomavirus agents.

Endoplasmic reticulum in viral infection

Virus exploits host cellular machinery to replicate and form new viral progeny and endoplasmic reticulum (ER) plays central role in the interplay between virus and host cell. Here I will discuss how cellular functions of ER being utilized by viruses from different families during different stages of pathogenesis. Flow of knowledge related to this area of research based on interdisciplinary approach, using biochemical and cell biological assays coupled with advanced microscopy strategies, is pushing our understanding of the virus-ER interaction during infection to the next level.

SV40 Hijacks Cellular Transport, Membrane Penetration, and Disassembly Machineries to Promote Infection

During entry, a virus must be transported through the endomembrane system of the host cell, penetrate a cellular membrane, and undergo capsid disassembly, to reach the cytosol and often the nucleus in order to cause infection. To do so requires the virus to coordinately exploit the action of cellular membrane transport, penetration, and disassembly machineries. How this is accomplished remains enigmatic for many viruses, especially for viruses belonging to the nonenveloped virus family. In this review, we present the current model describing infectious entry of the nonenveloped polyomavirus (PyV) SV40. Insights from SV40 entry are likely to provide strategies to combat PyV-induced diseases, and to illuminate cellular trafficking, membrane transport, and disassembly mechanisms.

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.

JC Polyomavirus Entry by Clathrin-Mediated Endocytosis Is Driven by β-Arrestin

JC polyomavirus (JCPyV) establishes a persistent, lifelong, asymptomatic infection within the kidney of the majority of the human population. Under conditions of severe immunosuppression or immune modulation, JCPyV can reactivate in the central nervous system (CNS) and cause progressive multifocal leukoencephalopathy (PML), a fatal demyelinating disease. Initiation of infection is mediated through viral attachment to α2,6-sialic acid-containing lactoseries tetrasaccharide c (LSTc) on the surface of host cells. JCPyV internalization is dependent on serotonin 5-hydroxytryptamine subfamily 2 receptors (5-HT₂Rs), and entry is thought to occur by clathrin-mediated endocytosis (CME). However, the JCPyV entry process and the cellular factors involved in viral internalization remain poorly understood. Treatment of cells with small-molecule chemical inhibitors and RNA interference of 5-HT₂R endocytic machinery, including β-arrestin, clathrin, AP2, and dynamin, significantly reduced JCPyV infection. However, infectivity of the polyomavirus simian virus 40 (SV40) was not affected by CME-specific treatments. Inhibition of clathrin or β-arrestin specifically reduced JCPyV internalization but did not affect viral attachment. Furthermore, mutagenesis of a β-arrestin binding domain (Ala-Ser-Lys) within the intracellular C terminus of 5-HT_2AR severely diminished internalization and infection, suggesting that β-arrestin interactions with 5-HT_2AR are critical for JCPyV infection and entry. These conclusions illuminate key host factors that regulate clathrin-mediated endocytosis of JCPyV, which is necessary for viral internalization and productive infection.IMPORTANCE Viruses usurp cellular factors to invade host cells. Activation and utilization of these proteins upon initiation of viral infection are therefore required for productive infection and resultant viral disease. The majority of healthy individuals are asymptomatically infected by JC polyomavirus (JCPyV), but if the host immune system is compromised, JCPyV can cause progressive multifocal leukoencephalopathy (PML), a rare, fatal, demyelinating disease. Individuals infected with HIV or taking prolonged immunomodulatory therapies have a heightened risk for developing PML. The cellular proteins and pathways utilized by JCPyV to mediate viral entry are poorly understood. Our findings further characterize how JCPyV utilizes the clathrin-mediated endocytosis pathway to invade host cells. We have identified specific components of this pathway that are necessary for the viral entry process and infection. Collectively, the conclusions increase our understanding of JCPyV infection and pathogenesis and may contribute to the future development of novel therapeutic strategies for PML.

Oligoarginine-Bearing Tandem Repeat Penetration-Accelerating Sequence Delivers Protein to Cytosol via Caveolae-Mediated Endocytosis

To facilitate the cytosolic delivery of larger molecules such as proteins, we developed a new cell-penetrating peptide sequence, named Pas2r12, consisting of a repeated Pas sequence (FFLIG-FFLIG) and d-dodeca-arginine (r12). This peptide significantly enhanced the cellular uptake and cytosolic release of enhanced green fluorescent protein and immunoglobulin G as cargos. We found that simply mixing Pas2r12 with cargos could generate cytosolic introducible forms. The cytosolic delivery of cargos by Pas2r12 was found to be an energy-requiring process, to rely on actin polymerization, and to be suppressed by caveolae-mediated endocytosis inhibitors (genistein and methyl-β-cyclodextrin) and small interfering RNA against caveolin-1. These results suggest that Pas2r12 enhances membrane penetration of cargos without the need for cross-linking and that caveolae-mediated endocytosis may be the route by which cytosolic delivery is enhanced.

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.

Dynein Engages and Disassembles Cytosol-Localized Simian Virus 40 To Promote Infection

During entry, polyomavirus (PyV) is endocytosed and sorts to the endoplasmic reticulum (ER), where it penetrates the ER membrane to reach the cytosol. From the cytosol, the virus moves to the nucleus to cause infection. How PyV is transported from the cytosol into the nucleus, a crucial infection step, is unclear. We found that upon reaching the cytosol, the archetypal PyV simian virus 40 (SV40) recruits the cytoplasmic dynein motor, which disassembles the viral particle. This reaction enables the resulting disassembled virus to enter the nucleus to promote infection. Our findings reveal how a cytosolic motor can be hijacked to impart conformational changes to a viral particle, a process essential for successful infection.IMPORTANCE How a nonenveloped virus successfully traffics from the cell surface to the nucleus to cause infection remains enigmatic in many instances. In the case of the nonenveloped PyV, the viral particle is sorted from the plasma membrane to the ER and then the cytosol, from which it enters the nucleus to promote infection. The molecular mechanism by which PyV reaches the nucleus from the cytosol is not entirely clear. Here we demonstrate that the prototype PyV SV40 recruits dynein upon reaching the cytosol. Importantly, this cellular motor disassembles the viral particle during cytosol-to-nucleus transport to cause infection.

How Simian Virus 40 Hijacks the Intracellular Protein Trafficking Pathway to Its Own Benefit … and Ours

Viruses efficiently transfer and express their genes in host cells and evolve to evade the host's defense responses. These properties render them highly attractive for use as gene delivery vectors in vaccines, gene, and immunotherapies. Among the viruses used as gene delivery vectors, the macaque polyomavirus Simian Virus 40 (SV40) is unique in its capacity to evade intracellular antiviral defense responses upon cell entry. We here describe the unique way by which SV40 particles deliver their genomes in the nucleus of permissive cells and how they prevent presentation of viral antigens to the host's immune system. The non-immunogenicity in its natural host is not only of benefit to the virus but also to us in developing effective SV40 vector-based treatments for today's major human diseases.

Interaction of the Mouse Polyomavirus Capsid Proteins with Importins Is Required for Efficient Import of Viral DNA into the Cell Nucleus

The mechanism used by mouse polyomavirus (MPyV) to overcome the crowded cytosol to reach the nucleus has not been fully elucidated. Here, we investigated the involvement of importin α/β1 mediated transport in the delivery of MPyV genomes into the nucleus. Interactions of the virus with importin β1 were studied by co-immunoprecipitation and proximity ligation assay. For infectivity and nucleus delivery assays, the virus and its capsid proteins mutated in the nuclear localization signals (NLSs) were prepared and produced. We found that at early times post infection, virions bound importin β1 in a time dependent manner with a peak of interactions at 6 h post infection. Mutation analysis revealed that only when the NLSs of both VP1 and VP2/3 were disrupted, virus did not bind efficiently to importin β1 and its infectivity remarkably decreased (by 80%). Nuclear targeting of capsid proteins was improved when VP1 and VP2 were co-expressed. VP1 and VP2 were effectively delivered into the nucleus, even when one of the NLS, either VP1 or VP2, was disrupted. Altogether, our results showed that MPyV virions can use VP1 and/or VP2/VP3 NLSs in concert or individually to bind importins to deliver their genomes into the cell nucleus.

Cleavage of the C-Terminal Fragment of Reovirus μ1 Is Required for Optimal Infectivity

The mammalian orthoreovirus (reovirus) outer capsid, which is composed of 200 μ1/σ3 heterohexamers and a maximum of 12 σ1 trimers, contains all of the proteins that are necessary for attaching to and entering host cells. Following attachment, reovirus is internalized by receptor-mediated endocytosis and acid-dependent cathepsin proteases degrade the σ3 protein. This process generates a metastable intermediate, called infectious subviral particle (ISVP), in which the μ1 membrane penetration protein is exposed. ISVPs undergo a second structural rearrangement to deposit the genome-containing core into the host cytoplasm. The conformationally altered particle is called ISVP*. ISVP-to-ISVP* conversion culminates in the release of μ1 N- and C-terminal fragments, μ1N and Φ, respectively. Released μ1N is thought to facilitate core delivery by generating size-selective pores within the endosomal membrane, whereas the precise role of Φ, particularly in the context of viral entry, is undefined. In this report, we characterize a recombinant reovirus that fails to cleave Φ from μ1 in vitro Φ cleavage, which is not required for ISVP-to-ISVP* conversion, enhances the disruption of liposomal membranes and facilitates the recruitment of ISVP*s to the site of pore formation. Moreover, the Φ cleavage-deficient strain initiates infection of host cells less efficiently than the parental strain. These results indicate that μ1N and Φ contribute to reovirus pore forming activity.IMPORTANCE Host membranes represent a physical barrier that prevents infection. To overcome this barrier, viruses utilize diverse strategies, such as membrane fusion or membrane disruption, to access internal components of the cell. These strategies are characterized by discrete protein-protein and protein-lipid interactions. The mammalian orthoreovirus (reovirus) outer capsid undergoes a series of well-defined conformational changes, which conclude with pore formation and delivery of the viral genetic material. In this report, we characterize the role of the small, reovirus-derived Φ peptide in pore formation. Φ cleavage from the outer capsid enhances membrane disruption and facilitates the recruitment of virions to membrane-associated pores. Moreover, Φ cleavage promotes the initiation of infection. Together, these results reveal an additional component of the reovirus pore forming apparatus and highlight a strategy for penetrating host membranes.

Identification of Rab18 as an Essential Host Factor for BK Polyomavirus Infection Using a Whole-Genome RNA Interference Screen

Polyomaviruses bind to a group of specific gangliosides on the plasma membrane of the cell prior to being endocytosed. They then follow a retrograde trafficking pathway to reach the endoplasmic reticulum (ER). The viruses begin to disassemble in the ER and then exit the ER and move to the nucleus. However, the details of intracellular trafficking between the endosome and the ER are largely unknown. By implementing a whole human genome small interfering RNA screen, we identified Rab18, syntaxin 18, and the NRZ complex as key components in endosome-ER trafficking of the human polyomavirus BKPyV. These results serve to further elucidate the route BKPyV takes from outside the cell to its site of replication in the nucleus.

BK polyomavirus (BKPyV) is a human pathogen first isolated in 1971. BKPyV infection is ubiquitous in the human population, with over 80% of adults worldwide being seropositive for BKPyV. BKPyV infection is usually asymptomatic; however, BKPyV reactivation in immunosuppressed transplant patients causes two diseases, polyomavirus-associated nephropathy and hemorrhagic cystitis. To establish a successful infection in host cells, BKPyV must travel in retrograde transport vesicles to reach the nucleus. To make this happen, BKPyV requires the cooperation of host cell proteins. To further identify host factors associated with BKPyV entry and intracellular trafficking, we performed a whole-genome small interfering RNA screen on BKPyV infection of primary human renal proximal tubule epithelial cells. The results revealed the importance of Ras-related protein Rab18 and syntaxin 18 for BKPyV infection. Our subsequent experiments implicated additional factors that interact with this pathway and suggest a more detailed model of the intracellular trafficking process, indicating that BKPyV reaches the endoplasmic reticulum (ER) lumen through a retrograde transport pathway between the late endosome and the ER.

IMPORTANCE Polyomaviruses bind to a group of specific gangliosides on the plasma membrane of the cell prior to being endocytosed. They then follow a retrograde trafficking pathway to reach the endoplasmic reticulum (ER). The viruses begin to disassemble in the ER and then exit the ER and move to the nucleus. However, the details of intracellular trafficking between the endosome and the ER are largely unknown. By implementing a whole human genome small interfering RNA screen, we identified Rab18, syntaxin 18, and the NRZ complex as key components in endosome-ER trafficking of the human polyomavirus BKPyV. These results serve to further elucidate the route BKPyV takes from outside the cell to its site of replication in the nucleus.

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.

Exploiting the kinesin-1 molecular motor to generate a virus membrane penetration site

Viruses exploit cellular machineries to penetrate a host membrane and cause infection, a process that remains enigmatic for non-enveloped viruses. Here we probe how the non-enveloped polyomavirus SV40 penetrates the endoplasmic reticulum (ER) membrane to reach the cytosol, a crucial infection step. We find that the microtubule-based motor kinesin-1 is recruited to the ER membrane by binding to the transmembrane J-protein B14. Strikingly, this motor facilitates SV40 ER-to-cytosol transport by constructing a penetration site on the ER membrane called a ‘focus'. Neither kinesin-2, kinesin-3 nor kinesin-5 promotes foci formation or infection. The specific use of kinesin-1 is due to its unique ability to select posttranslationally modified microtubules for cargo transport and thereby spatially restrict focus formation to the perinucleus. These findings support the idea of a ‘tubulin code' for motor-dependent trafficking and establish a distinct kinesin-1 function in which a motor is exploited to create a viral membrane penetration site.

How non-enveloped viruses cross host membranes is incompletely understood. Here, Ravindran et al. show that polyomavirus SV40 recruits kinesin-1 to construct a penetration site on the ER membrane.

Entry, infection, replication, and egress of human polyomaviruses: an update

Polyomaviruses (PyVs), belonging to the family Polyomaviridae, are a group of small, nonenveloped, double-stranded, circular DNA viruses widely distributed in the vertebrates. PyVs cause no apparent disease in adult laboratory mice but cause a wide variety of tumors when artificially inoculated into neonates or semipermissive animals. A few human PyVs, such as BK, JC, and Merkel cell PyVs, have been unequivocally linked to pathogenesis under conditions of immunosuppression. Infection is thought to occur early in life and persists for the lifespan of the host. Over evolutionary time scales, it appears that PyVs have slowly co-evolved with specific host animal lineages. Host cell surface glycoproteins and glycolipids seem to play a decisive role in the entry stage of viral infection and in channeling the virions to specific intracellular membrane-bound compartments and ultimately to the nucleus, where the genomes are replicated and packaged for release. Therefore the transport of the infecting virion or viral genome to this site of multiplication is an essential process in productive viral infection as well as in latent infection and transformation. This review summarizes the major findings related to the characterization of the nature of the interactions between PyV and host protein and their impact in host cell invasion.

[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

How Polyomaviruses Exploit the ERAD Machinery to Cause Infection

To infect cells, polyomavirus (PyV) traffics from the cell surface to the endoplasmic reticulum (ER) where it hijacks elements of the ER-associated degradation (ERAD) machinery to penetrate the ER membrane and reach the cytosol. From the cytosol, the virus transports to the nucleus, enabling transcription and replication of the viral genome that leads to lytic infection or cellular transformation. How PyV exploits the ERAD machinery to cross the ER membrane and access the cytosol, a decisive infection step, remains enigmatic. However, recent studies have slowly unraveled many aspects of this process. These emerging insights should advance our efforts to develop more effective therapies against PyV-induced human diseases.

Opportunistic intruders: how viruses orchestrate ER functions to infect cells

Viruses exploit the functions of the endoplasmic reticulum (ER) to promote both early and later stages of their life cycle, including entry, translation, replication, assembly, morphogenesis and egress. This observation reveals a shared principle that underlies virus–host cell relationships.

Viral entry often requires disassembly of the incoming virus particle. This is best exemplified in the case of polyomavirus entry, in which ER-associated machineries are hijacked to disassemble the virus and promote entry to the cytosol en route to the nucleus.

Many enveloped viruses, such as HIV and influenza virus, co-opt the ER-associated protein biosynthetic machinery to translate their genome and produce structural proteins that are necessary for the formation of virus particles and non-structural proteins that are essential during genome replication.

Replication of the viral genome, particularly for positive-sense RNA ((+)RNA) viruses including hepatitis C virus (HCV), dengue virus (DENV) and West Nile virus (WNV), occurs in virus-induced membranous structures that are most often derived from the ER. The formation of these structures requires morphological changes to the ER membrane, involving membrane rearrangements that are induced by viral non-structural proteins that are targeted to the ER.

As virus assembly is often coupled to genome replication, the assembly process frequently relies on the ER membrane. This strategy is seen for both RNA and DNA viruses.

Morphogenesis of assembled virus particles can also take advantage of the ER. This is best observed in the non-enveloped rotavirus, for which a transient enveloped intermediate is converted to the mature and infectious particle in the lumen of the ER.

After maturation in the ER, progeny virus particles egress the host through the ER-dependent secretory pathway, which provides a physical conduit to the extracellular environment.

The overall observations that the ER actively promotes all steps of viral infection have therapeutic implications. The development of chemical inhibitors of selective ER-associated components is emerging as a potential avenue of antiviral therapy, provided that these inhibitors have minimal toxicity to the host cell.

Many host structures are vital for viral infection and the endoplasmic reticulum (ER), in particular, is essential. In this Review, Tsai and colleagues highlight examples of subversion of the ER by diverse viruses to promote all stages of their life cycle, from entry to egress.

Viruses subvert the functions of their host cells to replicate and form new viral progeny. The endoplasmic reticulum (ER) has been identified as a central organelle that governs the intracellular interplay between viruses and hosts. In this Review, we analyse how viruses from vastly different families converge on this unique intracellular organelle during infection, co-opting some of the endogenous functions of the ER to promote distinct steps of the viral life cycle from entry and replication to assembly and egress. The ER can act as the common denominator during infection for diverse virus families, thereby providing a shared principle that underlies the apparent complexity of relationships between viruses and host cells. As a plethora of information illuminating the molecular and cellular basis of virus–ER interactions has become available, these insights may lead to the development of crucial therapeutic agents.

Endoplasmic Reticulum-Associated Degradation and Lipid Homeostasis

The endoplasmic reticulum is the port of entry for proteins into the secretory pathway and the site of synthesis for several important lipids, including cholesterol, triacylglycerol, and phospholipids. Protein production within the endoplasmic reticulum is tightly regulated by a cohort of resident machinery that coordinates the folding, modification, and deployment of secreted and integral membrane proteins. Proteins failing to attain their native conformation are degraded through the endoplasmic reticulum-associated degradation (ERAD) pathway via a series of tightly coupled steps: substrate recognition, dislocation, and ubiquitin-dependent proteasomal destruction. The same ERAD machinery also controls the flux through various metabolic pathways by coupling the turnover of metabolic enzymes to the levels of key metabolites. We review the current understanding and biological significance of ERAD-mediated regulation of lipid metabolism in mammalian cells.

Incoming human papillomavirus type 16 genome resides in a vesicular compartment throughout mitosis

During the entry process, the human papillomavirus (HPV) capsid is trafficked to the trans-Golgi network (TGN), whereupon it enters the nucleus during mitosis. We previously demonstrated that the minor capsid protein L2 assumes a transmembranous conformation in the TGN. Here we provide evidence that the incoming viral genome dissociates from the TGN and associates with microtubules after the onset of mitosis. Deposition onto mitotic chromosomes is L2-mediated. Using differential staining of an incoming viral genome by small molecular dyes in selectively permeabilized cells, nuclease protection, and flotation assays, we found that HPV resides in a membrane-bound vesicle until mitosis is completed and the nuclear envelope has reformed. As a result, expression of the incoming viral genome is delayed. Taken together, these data provide evidence that HPV has evolved a unique strategy for delivering the viral genome to the nucleus of dividing cells. Furthermore, it is unlikely that nuclear vesicles are unique to HPV, and thus we may have uncovered a hitherto unrecognized cellular pathway that may be of interest for future cell biological studies.

Principles of Virus Uncoating: Cues and the Snooker Ball

Viruses are spherical or complex shaped carriers of proteins, nucleic acids and sometimes lipids and sugars. They are metastable and poised for structural changes. These features allow viruses to communicate with host cells during entry, and to release the viral genome, a process known as uncoating. Studies have shown that hundreds of host factors directly or indirectly support this process. The cell provides molecules that promote stepwise virus uncoating, and direct the virus to the site of replication. It acts akin to a snooker player who delivers accurate and timely shots (cues) to the ball (virus) to score. The viruses, on the other hand, trick (snooker) the host, hijack its homeostasis systems, and dampen innate immune responses directed against danger signals. In this review, we discuss how cellular cues, facilitators, and built‐in viral mechanisms promote uncoating. Cues come from receptors, enzymes and chemicals that act directly on the virus particle to alter its structure, trafficking and infectivity. Facilitators are defined as host factors that are involved in processes which indirectly enhance entry or uncoating. Unraveling the mechanisms of virus uncoating will continue to enhance understanding of cell functions, and help counteracting infections with chemicals and vaccines.

Lipid Membranes Facilitate Conformational Changes Required for Reovirus Cell Entry

Cellular entry of nonenveloped and enveloped viruses is often accompanied by dramatic conformational changes within viral structural proteins. These rearrangements are triggered by a variety of mechanisms, such as low pH, virus-receptor interactions, and virus-host chaperone interactions. Reoviruses, a model system for entry of nonenveloped viruses, undergo a series of disassembly steps within the host endosome. One of these steps, infectious subviral particle (ISVP)-to-ISVP* conversion, is necessary for delivering the genome-containing viral core into host cells, but the physiological trigger that mediates ISVP-to-ISVP* conversion during cell entry is unknown. Structural studies of the reovirus membrane penetration protein, μ1, predict that interactions between μ1 and negatively charged lipid head groups may promote ISVP* formation; however, experimental evidence for this idea is lacking. Here, we show that the presence of polyanions (SO4(2-) and HPO4(2-)) or lipids in the form of liposomes facilitates ISVP-to-ISVP* conversion. The requirement for charged lipids appears to be selective, since phosphatidylcholine and phosphatidylethanolamine promoted ISVP* formation, whereas other lipids, such as sphingomyelin and sulfatide, either did not affect ISVP* formation or prevented ISVP* formation. Thus, our work provides evidence that interactions with membranes can function as a trigger for a nonenveloped virus to gain entry into host cells.

IMPORTANCE

Cell entry, a critical stage in the virus life cycle, concludes with the delivery of the viral genetic material across host membranes. Regulated structural transitions within nonenveloped and enveloped viruses are necessary for accomplishing this step; these conformational changes are predominantly triggered by low pH and/or interactions with host proteins. In this work, we describe a previously unknown trigger, interactions with lipid membranes, which can induce the structural rearrangements required for cell entry. This mechanism operates during entry of mammalian orthoreoviruses. We show that interactions between reovirus entry intermediates and lipid membranes devoid of host proteins promote conformational changes within the viral outer capsid that lead to membrane penetration. Thus, this work illustrates a novel strategy that nonenveloped viruses can use to gain access into cells and how viruses usurp disparate host factors to initiate infection.