Reovirus apoptosis and virulence are regulated by host cell membrane penetration efficiency

Mammalian orthoreovirus can exit cells in extracellular vesicles

Several egress pathways have been defined for many viruses. Among these pathways, extracellular vesicles (EVs) have been shown to function as vehicles of non-lytic viral egress. EVs are heterogenous populations of membrane-bound structures released from cells as a form of intercellular communication. EV-mediated viral egress may enable immune evasion and collective viral transport. Strains of nonenveloped mammalian orthoreovirus (reovirus) differ in cell lysis phenotypes, with T3D disrupting cell membranes more efficiently than T1L. However, mechanisms of reovirus egress and the influence of transport strategy on infection are only partially understood. To elucidate reovirus egress mechanisms, we infected murine fibroblasts (L cells) and non-polarized human colon epithelial (Caco-2) cells with T1L or T3D reovirus and enriched cell culture supernatants for large EVs, medium EVs, small EVs, and free reovirus. We found that both reovirus strains exit cells in association with large and medium EVs and as free virus particles, and that EV-enriched fractions are infectious. While reovirus visually associates with large and medium EVs, only medium EVs offer protection from antibody-mediated neutralization. EV-mediated protection from neutralization is virus strain- and cell type-specific, as medium EVs enriched from L cell supernatants protect T1L and T3D, while medium EVs enriched from Caco-2 cell supernatants largely fail to protect T3D and only protect T1L efficiently. Using genetically barcoded reovirus, we provide evidence that large and medium EVs can convey multiple particles to recipient cells. Finally, T1L or T3D infection increases the release of all EV sizes from L cells. Together, these findings suggest that in addition to exiting cells as free particles, reovirus promotes egress from distinct cell types in association with large and medium EVs during lytic or non-lytic infection, a mode of exit that can mediate multiparticle infection and, in some cases, protection from antibody neutralization.

Proteasome activity is required for reovirus entry into cells

IMPORTANCE

Due to their limited genetic capacity, viruses are reliant on multiple host systems to replicate successfully. Mammalian orthoreovirus (reovirus) is commonly used as a model system for understanding host-virus interactions. In this study, we identify that the proteasome system, which is critical for cellular protein turnover, affects reovirus entry. Inhibition of the proteasome using a chemical inhibitor blocks reovirus uncoating. Blocking these events reduces subsequent replication of the virus. This work identifies that additional host factors control reovirus entry.

Mammalian orthoreovirus can exit cells in extracellular vesicles

Several egress pathways have been defined for many viruses. Among these pathways, extracellular vesicles (EVs) have been shown to function as vehicles of non-lytic viral egress. EVs are heterogenous populations of membrane-bound structures released from cells as a form of intercellular communication. EV-mediated viral egress may enable immune evasion and collective viral transport. Strains of nonenveloped mammalian orthoreovirus (reovirus) differ in cell lysis phenotypes, with T3D disrupting cell membranes more efficiently than T1L. However, mechanisms of reovirus egress and the influence of transport strategy on infection are only partially understood. To elucidate reovirus egress mechanisms, we infected murine fibroblasts (L cells) and non-polarized human colon epithelial (Caco-2) cells with T1L or T3D reovirus and enriched cell culture supernatants for large EVs, medium EVs, small EVs, and free reovirus. We found that both reovirus strains exit cells in association with large and medium EVs and as free virus particles, and that EV-enriched fractions are infectious. While reovirus visually associates with large and medium EVs, only medium EVs offer protection from antibody-mediated neutralization. EV-mediated protection from neutralization is virus strain- and cell type-specific, as medium EVs enriched from L cell supernatants protect T1L and T3D, while medium EVs enriched from Caco-2 cell supernatants largely fail to protect T3D and only protect T1L efficiently. Using genetically barcoded reovirus, we provide evidence that large and medium EVs can convey multiple particles to recipient cells. Finally, T1L or T3D infection increases the release of all EV sizes from L cells. Together, these findings suggest that in addition to exiting cells as free particles, reovirus promotes egress from distinct cell types in association with large and medium EVs during lytic or non-lytic infection, a mode of exit that can mediate multiparticle infection and, in some cases, protection from antibody neutralization.

Cell Killing by Reovirus: Mechanisms and Consequences

Infection of host cells by mammalian reovirus in culture or in tissues of infected animals results in cell death. Cell death of infected neurons and myocytes contributes to the pathogenesis of reovirus-induced encephalitis and myocarditis in a newborn mouse model. Thus, reovirus-induced cell death has been used to investigate the basis of viral disease. Depending on the cell type, infection of host cells by reovirus results in one of two forms of cell death-apoptosis and necroptosis. In addition to the obvious differences in how these two forms of cell death are executed, the mechanisms by which reovirus infection initiates and transduces signals that lead to each of these types of cell death are distinct. In this review, we discuss how apoptosis and necroptosis are triggered by events at different stages of infection. We also describe how innate immune recognition of reovirus genomic material and type I interferon signaling pathways connect with the core components of the apoptosis and necroptosis machinery. The impact of different cell death mediators on viral pathogenesis and the potential of reovirus as an oncolytic vector are also outlined.

MicroRNAs as immune regulators and biomarkers in tuberculosis

Tuberculosis (TB), which is caused by Mycobacterium tuberculosis (Mtb), is one of the most lethal infectious disease worldwide, and it greatly affects human health. Some diagnostic and therapeutic methods are available to effectively prevent and treat TB; however, only a few systematic studies have described the roles of microRNAs (miRNAs) in TB. Combining multiple clinical datasets and previous studies on Mtb and miRNAs, we state that pathogens can exploit interactions between miRNAs and other biomolecules to avoid host mechanisms of immune-mediated clearance and survive in host cells for a long time. During the interaction between Mtb and host cells, miRNA expression levels are altered, resulting in the changes in the miRNA-mediated regulation of host cell metabolism, inflammatory responses, apoptosis, and autophagy. In addition, differential miRNA expression can be used to distinguish healthy individuals, patients with TB, and patients with latent TB. This review summarizes the roles of miRNAs in immune regulation and their application as biomarkers in TB. These findings could provide new opportunities for the diagnosis and treatment of TB.

Early Events in Reovirus Infection Influence Induction of Innate Immune Response

Mammalian orthoreovirus (reovirus) is a double-stranded RNA (dsRNA) virus which encapsidates its 10 genome segments within a double-layered viral particle. Reovirus infection triggers an antiviral response in host cells which serves to limit viral replication. This antiviral response is initiated by recognition of the incoming viral genome by host sensors present in the cytoplasm. However, how host sensors gain access to the reovirus genome is unclear, as this dsRNA is protected by the viral particle proteins throughout infection. To initiate infection, reovirus particles are endocytosed and the outer viral particle layer is disassembled through the action of host proteases. This disassembly event is required for viral escape into the cytoplasm to begin replication. We show that endosomal proteases are required even late in infection, when disassembly is complete, to induce an immune response to reovirus. Additionally, counter to dogma, our data demonstrate that at least some viral dsRNA genome is exposed and detectable during entry. We hypothesize that some proportion of reovirus particles remain trapped within endosomes, allowing for the breakdown of these particles and release of their genome. We show that rapidly uncoating mutants escape the endosome more rapidly and induce a diminished immune response. Further, we show that particles entering through dynamin-independent pathways evade detection by host sensors. Overall, our data provide new insight into how genomes from entering reovirus particles are detected by host cells. IMPORTANCE Viruses must infect host cells to replicate, often killing the host cell in the process. However, hosts can activate defenses to limit viral replication and protect the organism. To trigger these host defenses to viral infections, host cells must first recognize that they are infected. Mammalian orthoreovirus (reovirus) is a model system used to study host-virus interactions. This study identifies aspects of host and virus biology which determine the capacity of host cells to detect infection. Notably, entry of reovirus into host cells plays a critical role in determining the magnitude of immune response triggered during infection. Mutants of reovirus which can enter cells more rapidly are better at avoiding detection by the host. Additionally, reovirus can enter cells through multiple routes. Entry through some of these routes also helps reovirus evade detection.

Nanomaterials for virus sensing and tracking

The effect of the on-going COVID-19 pandemic on global healthcare systems has underlined the importance of timely and cost-effective point-of-care diagnosis of viruses. The need for ultrasensitive easy-to-use platforms has culminated in an increased interest for rapid response equipment-free alternatives to conventional diagnostic methods such as polymerase chain reaction, western-blot assay, etc. Furthermore, the poor stability and the bleaching behavior of several contemporary fluorescent reporters is a major obstacle in understanding the mechanism of viral infection thus retarding drug screening and development. Owing to their extraordinary surface-to-volume ratio as well as their quantum confinement and charge transfer properties, nanomaterials are desirable additives to sensing and imaging systems to amplify their signal response as well as temporal resolution. Their large surface area promotes biomolecular integration as well as efficacious signal transduction. Due to their hole mobility, photostability, resistance to photobleaching, and intense brightness, nanomaterials have a considerable edge over organic dyes for single virus tracking. This paper reviews the state-of-the-art of combining carbon-allotrope, inorganic and organic-based nanomaterials with virus sensing and tracking methods, starting with the impact of human pathogenic viruses on the society. We address how different nanomaterials can be used in various virus sensing platforms (e.g. lab-on-a-chip, paper, and smartphone-based point-of-care systems) as well as in virus tracking applications. We discuss the enormous potential for the use of nanomaterials as simple, versatile, and affordable tools for detecting and tracing viruses infectious to humans, animals, plants as well as bacteria. We present latest examples in this direction by emphasizing major advantages and limitations.

Reovirus Activated Cell Death Pathways

Mammalian orthoreoviruses (ReoV) are non-enveloped viruses with segmented double-stranded RNA genomes. In humans, ReoV are generally considered non-pathogenic, although members of this family have been proven to cause mild gastroenteritis in young children and may contribute to the development of inflammatory conditions, including Celiac disease. Because of its low pathogenic potential and its ability to efficiently infect and kill transformed cells, the ReoV strain Type 3 Dearing (T3D) is clinical trials as an oncolytic agent. ReoV manifests its oncolytic effects in large part by infecting tumor cells and activating programmed cell death pathways (PCDs). It was previously believed that apoptosis was the dominant PCD pathway triggered by ReoV infection. However, new studies suggest that ReoV also activates other PCD pathways, such as autophagy, pyroptosis, and necroptosis. Necroptosis is a caspase-independent form of PCD reliant on receptor-interacting serine/threonine-protein kinase 3 (RIPK3) and its substrate, the pseudokinase mixed-lineage kinase domain-like protein (MLKL). As necroptosis is highly inflammatory, ReoV-induced necroptosis may contribute to the oncolytic potential of this virus, not only by promoting necrotic lysis of the infected cell, but also by inflaming the surrounding tumor microenvironment and provoking beneficial anti-tumor immune responses. In this review, we summarize our current understanding of the ReoV replication cycle, the known and potential mechanisms by which ReoV induces PCD, and discuss the consequences of non-apoptotic cell death—particularly necroptosis—to ReoV pathogenesis and oncolysis.

Mechanisms of Cell Entry by dsRNA Viruses: Insights for Efficient Delivery of dsRNA and Tools for Improved RNAi-Based Pest Control

While RNAi is often heralded as a promising new strategy for insect pest control, a major obstacle that still remains is the efficient delivery of dsRNA molecules within the cells of the targeted insects. However, it seems overlooked that dsRNA viruses already have developed efficient strategies for transport of dsRNA molecules across tissue barriers and cellular membranes. Besides protecting their dsRNA genomes in a protective shell, dsRNA viruses also display outer capsid layers that incorporate sophisticated mechanisms to disrupt the plasma membrane layer and to translocate core particles (with linear dsRNA genome fragments) within the cytoplasm. Because of the perceived efficiency of the translocation mechanism, it is well worth analyzing in detail the molecular processes that are used to achieve this feat. In this review, the mechanism of cell entry by dsRNA viruses belonging to the Reoviridae family is discussed in detail. Because of the large amount of progress in mammalian versus insect models, the mechanism of infections of reoviruses in mammals (orthoreoviruses, rotaviruses, orbiviruses) will be treated as a point of reference against which infections of reoviruses in insects (orbiviruses in midges, plant viruses in hemipterans, insect-specific cypoviruses in lepidopterans) will be compared. The goal of this discussion is to uncover the basic principles by which dsRNA viruses cross tissue barriers and translocate their cargo to the cellular cytoplasm; such knowledge subsequently can be incorporated into the design of dsRNA virus-based viral-like particles for optimal delivery of RNAi triggers in targeted insect pests.

Apoptosis induced by SARS-CoV-2: can we target it?

Some viruses are known to be associated with increased apoptosis. Apoptotic cell death triggered by these viruses has a complex role in host antiviral immunity, and might facilitate the viral clearance or act as a mechanism for virus-induced tissue injury and disease progression. The induction of apoptosis is a hallmark of SARS-CoV-2 infection. Accumulating evidence suggests that there is a direct relationship between apoptosis rate and COVID‐19 pathogenicity/severity. Targeting virus-induced apoptosis could be a promising strategy in the treatment of SARS-CoV-2 virus infection.

Control of Capsid Transformations during Reovirus Entry

Mammalian orthoreovirus (reovirus), a dsRNA virus with a multilayered capsid, serves as a model system for studying the entry of similar viruses. The outermost layer of this capsid undergoes processing to generate a metastable intermediate. The metastable particle undergoes further remodeling to generate an entry-capable form that delivers the genome-containing inner capsid, or core, into the cytoplasm. In this review, we highlight capsid proteins and the intricacies of their interactions that control the stability of the capsid and consequently impact capsid structural changes that are prerequisites for entry. We also discuss a novel proviral role of host membranes in promoting capsid conformational transitions. Current knowledge gaps in the field that are ripe for future investigation are also outlined.

Loss of IKK Subunits Limits NF-κB Signaling in Reovirus-Infected Cells

Viruses commonly antagonize innate immune pathways that are primarily driven by nuclear factor kappa B (NF-κB), interferon regulatory factor (IRF), and the signal transducer and activator of transcription proteins (STAT) family of transcription factors. Such a strategy allows viruses to evade immune surveillance and maximize their replication. Using an unbiased transcriptome sequencing (RNA-seq)-based approach to measure gene expression induced by transfected viral genomic RNA (vgRNA) and reovirus infection, we discovered that mammalian reovirus inhibits host cell innate immune signaling. We found that, while vgRNA and reovirus infection both induce a similar IRF-dependent gene expression program, gene expression driven by the NF-κB family of transcription factors is lower in infected cells. Potent agonists of NF-κB such as tumor necrosis factor alpha (TNF-α) and vgRNA failed to induce NF-κB-dependent gene expression in infected cells. We demonstrate that NF-κB signaling is blocked due to loss of critical members of the inhibitor of kappa B kinase (IKK) complex, NF-κB essential modifier (NEMO), and IKKβ. The loss of the IKK complex components prevents nuclear translocation and phosphorylation of NF-κB, thereby preventing gene expression. Our study demonstrates that reovirus infection selectively blocks NF-κB, likely to counteract its antiviral effects and promote efficient viral replication.IMPORTANCE Host cells mount a response to curb virus replication in infected cells and prevent spread of virus to neighboring, as yet uninfected, cells. The NF-κB family of proteins is important for the cell to mediate this response. In this study, we show that in cells infected with mammalian reovirus, NF-κB is inactive. Further, we demonstrate that NF-κB is rendered inactive because virus infection results in reduced levels of upstream intermediaries (called IKKs) that are needed for NF-κB function. Based on previous evidence that active NF-κB limits reovirus infection, we conclude that inactivating NF-κB is a viral strategy to produce a cellular environment that is favorable for virus replication.

Enhanced Killing of Triple-Negative Breast Cancer Cells by Reassortant Reovirus and Topoisomerase Inhibitors

Breast cancer is the second leading cause of cancer-related deaths in women in the United States. Triple-negative breast cancer constitutes a subset of breast cancer that is associated with higher rates of relapse, decreased survival, and limited therapeutic options for patients afflicted with this type of breast cancer. Mammalian orthoreovirus (reovirus) selectively infects and kills transformed cells, and a serotype 3 reovirus is in clinical trials to assess its efficacy as an oncolytic agent against several cancers. It is unclear if reovirus serotypes differentially infect and kill triple-negative breast cancer cells and if reovirus-induced cytotoxicity of breast cancer cells can be enhanced by modulating the activity of host molecules and pathways. Here, we generated reassortant reoviruses by forward genetics with enhanced infective and cytotoxic properties in triple-negative breast cancer cells. From a high-throughput screen of small-molecule inhibitors, we identified topoisomerase inhibitors as a class of drugs that enhance reovirus infectivity and cytotoxicity of triple-negative breast cancer cells. Treatment of triple-negative breast cancer cells with topoisomerase inhibitors activates DNA damage response pathways, and reovirus infection induces robust production of type III, but not type I, interferon (IFN). Although type I and type III IFNs can activate STAT1 and STAT2, triple-negative breast cancer cellular proliferation is only negatively affected by type I IFN. Together, these data show that reassortant viruses with a novel genetic composition generated by forward genetics in combination with topoisomerase inhibitors more efficiently infect and kill triple-negative breast cancer cells.IMPORTANCE Patients afflicted by triple-negative breast cancer have decreased survival and limited therapeutic options. Reovirus infection results in cell death of a variety of cancers, but it is unknown if different reovirus types lead to triple-negative breast cancer cell death. In this study, we generated two novel reoviruses that more efficiently infect and kill triple-negative breast cancer cells. We show that infection in the presence of DNA-damaging agents enhances infection and triple-negative breast cancer cell killing by reovirus. These data suggest that a combination of a genetically engineered oncolytic reovirus and topoisomerase inhibitors may provide a potent therapeutic option for patients afflicted with triple-negative breast cancer.

Cell Entry-Independent Role for the Reovirus μ1 Protein in Regulating Necroptosis and the Accumulation of Viral Gene Products

The reovirus outer capsid protein μ1 regulates cell death in infected cells. To distinguish between the roles of incoming, capsid-associated, and newly synthesized μ1, we used small interfering RNA (siRNA)-mediated knockdown. Loss of newly synthesized μ1 protein does not affect apoptotic cell death in HeLa cells but enhances necroptosis in L929 cells. Knockdown of μ1 also affects aspects of viral replication. We found that, while μ1 knockdown results in diminished release of infectious viral progeny from infected cells, viral minus-strand RNA, plus-strand RNA, and proteins that are not targeted by the μ1 siRNA accumulate to a greater extent than in control siRNA-treated cells. Furthermore, we observed a decrease in sensitivity of these viral products to inhibition by guanidine hydrochloride (GuHCl) (which targets minus-strand synthesis to produce double-stranded RNA) when μ1 is knocked down. Following μ1 knockdown, cell death is also less sensitive to treatment with GuHCl. Our studies suggest that the absence of μ1 allows enhanced transcriptional activity of newly synthesized cores and the consequent accumulation of viral gene products. We speculate that enhanced accumulation and detection of these gene products due to μ1 knockdown potentiates receptor-interacting protein 3 (RIP3)-dependent cell death.IMPORTANCE We used mammalian reovirus as a model to study how virus infections result in cell death. Here, we sought to determine how viral factors regulate cell death. Our work highlights a previously unknown role for the reovirus outer capsid protein μ1 in limiting the induction of a necrotic form of cell death called necroptosis. Induction of cell death by necroptosis requires the detection of viral gene products late in infection; μ1 limits cell death by this mechanism because it prevents excessive accumulation of viral gene products that trigger cell death.

Components of the Reovirus Capsid Differentially Contribute to Stability

The mammalian orthoreovirus (reovirus) outer capsid is composed of 200 μ1-σ3 heterohexamers and a maximum of 12 σ1 trimers. During cell entry, σ3 is degraded by luminal or intracellular proteases to generate the infectious subviral particle (ISVP). When ISVP formation is prevented, reovirus fails to establish a productive infection, suggesting proteolytic priming is required for entry. ISVPs are then converted to ISVP*s, which is accompanied by μ1 rearrangements. The μ1 and σ3 proteins confer resistance to inactivating agents; however, neither the impact on capsid properties nor the mechanism (or basis) of inactivation is fully understood. Here, we utilized T1L/T3D M2 and T3D/T1L S4 to investigate the determinants of reovirus stability. Both reassortants encode mismatched subunits. When μ1-σ3 were derived from different strains, virions resembled wild-type particles in structure and protease sensitivity. T1L/T3D M2 and T3D/T1L S4 ISVPs were less thermostable than wild-type ISVPs. In contrast, virions were equally susceptible to heating. Virion associated μ1 adopted an ISVP*-like conformation concurrent with inactivation; σ3 preserves infectivity by preventing μ1 rearrangements. Moreover, thermostability was enhanced by a hyperstable variant of μ1. Unlike the outer capsid, the inner capsid (core) was highly resistant to elevated temperatures. The dual layered architecture allowed for differential sensitivity to inactivating agents.IMPORTANCE Nonenveloped and enveloped viruses are exposed to the environment during transmission to a new host. Protein-protein and/or protein-lipid interactions stabilize the particle and protect the viral genome. Mammalian orthoreovirus (reovirus) is composed of two concentric, protein shells. The μ1 and σ3 proteins form the outer capsid; contacts between neighboring subunits are thought to confer resistance to inactivating agents. We further investigated the determinants of reovirus stability. The outer capsid was disrupted concurrent with the loss of infectivity; virion associated μ1 rearranged into an altered conformation. Heat sensitivity was controlled by σ3; however, particle integrity was enhanced by a single μ1 mutation. In contrast, the inner capsid (core) displayed superior resistance to heating. These findings reveal structural components that differentially contribute to reovirus stability.

Protein Mismatches Caused by Reassortment Influence Functions of the Reovirus Capsid

Following attachment to host receptors via σ1, reovirus particles are endocytosed and disassembled to generate infectious subvirion particles (ISVPs). ISVPs undergo conformational changes to form ISVP*, releasing σ1 and membrane-targeting peptides from the viral μ1 protein. ISVP* formation is required for delivery of the viral core into the cytoplasm for replication. We characterized the properties of T3D^F/T3D^CS1, an S1 gene monoreassortant between two laboratory isolates of prototype reovirus strain T3D: T3D^F and T3D^C T3D^F/T3D^CS1 is poorly infectious. This deficiency is a consequence of inefficient encapsidation of S1-encoded σ1 on T3D^F/T3D^CS1 virions. Additionally, compared to T3D^F, T3D^F/T3D^CS1 undergoes ISVP-to-ISVP* conversion more readily, revealing an unexpected role for σ1 in regulating ISVP* formation. The σ1 protein is held within turrets formed by the λ2 protein. To test if the altered properties of T3D^F/T3D^CS1 are due to a mismatch between σ1 and λ2 proteins from T3D^F and T3D^C, properties of T3D^F/T3D^CL2 and T3D^F/T3D^CS1L2, which express a T3D^C-derived λ2, were compared. The presence of T3D^C λ2 allowed more efficient σ1 incorporation, producing particles that exhibit T3D^F-like infectivity. Compared to T3D^F, T3D^F/T3D^CL2 prematurely converts to ISVP*, uncovering a role for λ2 in regulating ISVP* formation. Importantly, a virus with matching σ1 and λ2 displayed a more regulated conversion to ISVP* than either T3D^F/T3D^CS1 or T3D^F/T3D^CL2. In addition to identifying new regulators of ISVP* formation, our results highlight that protein mismatches produced by reassortment can alter virus assembly and thereby influence subsequent functions of the virus capsid.IMPORTANCE Cells coinfected with viruses that possess a multipartite or segmented genome reassort to produce progeny viruses that contain a combination of gene segments from each parent. Reassortment places new pairs of genes together, generating viruses in which mismatched proteins must function together. To test if such forced pairing of proteins that form the virus shell or capsid alters the function of the particle, we investigated properties of reovirus variants in which the σ1 attachment protein and the λ2 protein that anchors σ1 on the particle are mismatched. Our studies demonstrate that a σ1-λ2 mismatch produces particles with lower levels of encapsidated σ1, consequently decreasing virus attachment and infectivity. The mismatch between σ1 and λ2 also altered the capacity of the viral capsid to undergo conformational changes required for cell entry. These studies reveal new functions of reovirus capsid proteins and illuminate both predictable and novel implications of reassortment.

Reovirus-Induced Apoptosis in the Intestine Limits Establishment of Enteric Infection

Several viruses induce intestinal epithelial cell death during enteric infection. However, it is unclear whether proapoptotic capacity promotes or inhibits replication in this tissue. We infected mice with two reovirus strains that infect the intestine but differ in the capacity to alter immunological tolerance to new food antigen. Infection with reovirus strain T1L, which induces an inflammatory immune response to fed antigen, is prolonged in the intestine, whereas T3D-RV, which does not induce this response, is rapidly cleared from the intestine. Compared with T1L, T3D-RV infection triggered apoptosis of intestinal epithelial cells and subsequent sloughing of dead cells into the intestinal lumen. We conclude that the infection advantage of T1L derives from its capacity to subvert host restriction by epithelial cell apoptosis, providing a possible mechanism by which T1L enhances inflammatory signals during antigen feeding. Using a panel of T1L × T3D-RV reassortant viruses, we identified the viral M1 and M2 gene segments as determinants of reovirus-induced apoptosis in the intestine. Expression of the T1L M1 and M2 genes in a T3D-RV background was sufficient to limit epithelial cell apoptosis and enhance viral infection to levels displayed by T1L. These findings define additional reovirus gene segments required for enteric infection of mice and illuminate the antiviral effect of intestinal epithelial cell apoptosis in limiting enteric viral infection. Viral strain-specific differences in the capacity to infect the intestine may be useful in identifying viruses capable of ameliorating tolerance to fed antigen in autoimmune conditions like celiac disease.IMPORTANCE Acute viral infections are thought to be cleared by the host with few lasting consequences. However, there may be much broader and long-lasting effects of viruses on immune homeostasis. Infection with reovirus, a common, nonpathogenic virus, triggers inflammation against innocuous food antigens, implicating this virus in the development of celiac disease, an autoimmune intestinal disorder triggered by exposure to dietary gluten. Using two reovirus strains that differ in the capacity to abrogate oral tolerance, we found that strain-specific differences in the capacity to replicate in the intestine inversely correlate with the capacity to induce apoptotic death of intestinal epithelial cells, providing a host-mediated process to restrict intestinal infection. This work contributes new knowledge about virus-host interactions in the intestine and establishes a foundation for future studies to define mechanisms by which viruses break oral tolerance in celiac disease.

Differential Delivery of Genomic Double-Stranded RNA Causes Reovirus Strain-Specific Differences in Interferon Regulatory Factor 3 Activation

Serotype 3 (T3) reoviruses induce substantially more type 1 interferon (IFN-I) secretion than serotype 1 (T1) strains. However, the mechanisms underlying differences in IFN-I production between T1 and T3 reoviruses remain undefined. Here, we found that differences in IFN-I production between T1 and T3 reoviruses correlate with activation of interferon regulatory factor 3 (IRF3), a key transcription factor for the production of IFN-I. T3 strain rsT3D activated IRF3 more rapidly and to a greater extent than the T1 strain rsT1L, in simian virus 40 (SV40) immortalized endothelial cells (SVECs). Differences in IRF3 activation between rsT1L and rsT3D were observed in the first hours of infection and were independent of de novo viral RNA and protein synthesis. NF-κB activation mirrored IRF3 activation, with rsT3D inducing more NF-κB activity than rsT1L. We also found that IRF3 and NF-κB are activated in a mitochondrial antiviral-signaling protein (MAVS)-dependent manner. rsT1L does not suppress IRF3 activation, as IRF3 phosphorylation could be induced in rsT1L-infected cells. Transfected rsT1L and rsT3D RNA induced IRF3 phosphorylation, indicating that genomic RNA from both strains has the capacity to activate IRF3. Finally, bypassing the normal route of reovirus entry by transfecting in vitro-generated viral cores revealed that rsT1L and rsT3D core particles induced equivalent IRF3 activation. Taken together, our findings indicate that entry-related events that occur after outer capsid disassembly, but prior to deposition of viral cores into the cytoplasm, influence the efficiency of IFN-I responses to reovirus. This work provides further insight into mechanisms by which nonenveloped viruses activate innate immune responses.IMPORTANCE Detection of viral nucleic acids by the host cell triggers type 1 interferon (IFN-I) responses, which are critical for containing and clearing viral infections. Viral RNA is sensed in the cytoplasm by cellular receptors that initiate signaling pathways, leading to the activation of interferon regulatory factor 3 (IRF3) and NF-κB, key transcription factors required for IFN-I induction. Serotype 3 (T3) reoviruses induce significantly more IFN-I than serotype 1 (T1) strains. In this work, we found that differences in IFN-I production by T1 and T3 reoviruses correlate with differential IRF3 activation. Differences in IRF3 activation are not caused by a blockade of the IRF3 activation by a T1 strain. Rather, differences in events during the late stages of viral entry determine the capacity of reovirus to activate host IFN-I responses. Together, our work provides insight into mechanisms of IFN-I induction by nonenveloped viruses.

Conflict in the Intracellular Lives of Endosymbionts and Viruses: A Mechanistic Look at Wolbachia-Mediated Pathogen-blocking

At the forefront of vector control efforts are strategies that leverage host-microbe associations to reduce vectorial capacity. The most promising of these efforts employs Wolbachia, a maternally transmitted endosymbiotic bacterium naturally found in 40% of insects. Wolbachia can spread through a population of insects while simultaneously inhibiting the replication of viruses within its host. Despite successes in using Wolbachia-transfected mosquitoes to limit dengue, Zika, and chikungunya transmission, the mechanisms behind pathogen-blocking have not been fully characterized. Firstly, we discuss how Wolbachia and viruses both require specific host-derived structures, compounds, and processes to initiate and maintain infection. There is significant overlap in these requirements, and infection with either microbe often manifests as cellular stress, which may be a key component of Wolbachia’s anti-viral effect. Secondly, we discuss the current understanding of pathogen-blocking through this lens of cellular stress and develop a comprehensive view of how the lives of Wolbachia and viruses are fundamentally in conflict with each other. A thorough understanding of the genetic and cellular determinants of pathogen-blocking will significantly enhance the ability of vector control programs to deploy and maintain effective Wolbachia-mediated control measures.

The Loop Formed by Residues 340 to 343 of Reovirus μ1 Controls Entry-Related Conformational Changes

Reovirus particles are covered with 200 μ1/σ3 heterohexamers. Following attachment to cell surface receptors, reovirus is internalized by receptor-mediated endocytosis. Within the endosome, particles undergo a series of stepwise disassembly events. First, the σ3 protector protein is degraded by cellular proteases to generate infectious subviral particles (ISVPs). Second, the μ1 protein rearranges into a protease-sensitive conformation to generate ISVP*s and releases two virus-encoded peptides, μ1N and Φ. The released peptides promote delivery of the genome-containing core by perforating the endosomal membrane. Thus, to establish a productive infection, virions must be stable in the environment but flexible to disassemble in response to the appropriate cellular cue. The reovirus outer capsid is stabilized by μ1 intratrimer, intertrimer, and trimer-core interactions. As a consequence of ISVP-to-ISVP* conversion, neighboring μ1 trimers unwind and separate. Located within the μ1 jelly roll β barrel domain, which is a known regulator of ISVP* formation, residues 340 to 343 form a loop and have been proposed to facilitate viral entry. To test this idea, we generated recombinant reoviruses that encoded deletions within this loop (Δ341 and Δ342). Both deletions destabilized the outer capsid. Notably, Δ342 impaired the viral life cycle; however, replicative fitness was restored by an additional change (V403A) within the μ1 jelly roll β barrel domain. In the Δ341 and Δ342 backgrounds, V403A also rescued defects in ISVP-to-ISVP* conversion. Together, these findings reveal a new region that regulates reovirus disassembly and how perturbing a metastable capsid can compromise replicative fitness.IMPORTANCE Capsids of nonenveloped viruses are composed of protein complexes that encapsulate, or form a shell around, nucleic acid. The protein-protein interactions that form this shell must be stable to protect the viral genome but also sufficiently flexible to disassemble during cell entry. Thus, capsids adopt conformations that undergo rapid disassembly in response to a specific cellular cue. In this work, we identify a new region within the mammalian orthoreovirus outer capsid that regulates particle stability. Amino acid deletions that destabilize this region impair the viral replication cycle. Nonetheless, replicative fitness is restored by a compensatory mutation that restores particle stability. Together, this work demonstrates the critical balance between assembling virions that are stable and maintaining conformational flexibility. Any factor that perturbs this balance has the potential to block a productive infection.

Serotype-Specific Killing of Large Cell Carcinoma Cells by Reovirus

Reovirus is under development as a therapeutic for numerous types of cancer. In contrast to other oncolytic viruses, the safety and efficacy of reovirus have not been improved through genetic manipulation. Here, we tested the oncolytic capacity of recombinant strains (rs) of prototype reovirus laboratory strains T1L and T3D (rsT1L and rsT3D, respectively) in a panel of non-small cell lung cancer (NSCLC) cell lines. We found that rsT1L was markedly more cytolytic than rsT3D in the large cell carcinoma cell lines tested, whereas killing of adenocarcinoma cell lines was comparable between rsT1L and rsT3D. Importantly, non-recombinant T1L and T3D phenocopied the kinetics and magnitude of cell death induced by recombinant strains. We identified gene segments L2, L3, and M1 as viral determinants of strain-specific differences cell killing of the large cell carcinoma cell lines. Together, these results indicate that recombinant reoviruses recapitulate the cell killing properties of non-recombinant, tissue culture-passaged strains. These studies provide a baseline for the use of reverse genetics with the specific objective of engineering more effective reovirus oncolytics. This work raises the possibility that type 1 reoviruses may have the capacity to serve as more effective oncolytics than type 3 reoviruses in some tumor types.

Viral RNA at Two Stages of Reovirus Infection Is Required for the Induction of Necroptosis

Necroptosis, a regulated form of necrotic cell death, requires the activation of the RIP3 kinase. Here, we identify that infection of host cells with reovirus can result in necroptosis. We find that necroptosis requires sensing of the genomic RNA within incoming virus particles via cytoplasmic RNA sensors to produce type I interferon (IFN). While these events that occur prior to the de novo synthesis of viral RNA are required for the induction of necroptosis, they are not sufficient. The induction of necroptosis also requires late stages of reovirus infection. Specifically, efficient synthesis of double-stranded RNA (dsRNA) within infected cells is required for necroptosis. These data indicate that viral RNA interfaces with host components at two different stages of infection to induce necroptosis. This work provides new molecular details about events in the viral replication cycle that contribute to the induction of necroptosis following infection with an RNA virus.IMPORTANCE An appreciation of how cell death pathways are regulated following viral infection may reveal strategies to limit tissue destruction and prevent the onset of disease. Cell death following virus infection can occur by apoptosis or a regulated form of necrosis known as necroptosis. Apoptotic cells are typically disposed of without activating the immune system. In contrast, necroptotic cells alert the immune system, resulting in inflammation and tissue damage. While apoptosis following virus infection has been extensively investigated, how necroptosis is unleashed following virus infection is understood for only a small group of viruses. Here, using mammalian reovirus, we highlight the molecular mechanism by which infection with a dsRNA virus results in necroptosis.

Reovirus μ1 Protein Affects Infectivity by Altering Virus-Receptor Interactions

Proteins that form the reovirus outer capsid play an active role in the entry of reovirus into host cells. Among these, the σ1 protein mediates attachment of reovirus particles to host cells via interaction with cell surface glycans or the proteinaceous receptor junctional adhesion molecule A (JAM-A). The μ1 protein functions to penetrate the host cell membrane to allow delivery of the genome-containing viral core particle into the cytoplasm to initiate viral replication. We demonstrate that a reassortant virus that expresses the M2 gene-encoded μ1 protein derived from prototype strain T3D in an otherwise prototype T1L background (T1L/T3DM2) infects cells more efficiently than parental T1L. Unexpectedly, the enhancement in infectivity of T1L/T3DM2 is due to its capacity to attach to cells more efficiently. We present genetic data implicating the central region of μ1 in altering the cell attachment property of reovirus. Our data indicate that the T3D μ1-mediated enhancement in infectivity of T1L is dependent on the function of σ1 and requires the expression of JAM-A. We also demonstrate that T1L/T3DM2 utilizes JAM-A more efficiently than T1L. These studies revealed a previously unknown relationship between two nonadjacent reovirus outer capsid proteins, σ1 and μ1.

IMPORTANCE

How reovirus attaches to host cells has been extensively characterized. Attachment of reovirus to host cells is mediated by the σ1 protein, and properties of σ1 influence the capacity of reovirus to target specific host tissues and produce disease. Here, we present new evidence indicating that the cell attachment properties of σ1 are influenced by the nature of μ1, a capsid protein that does not physically interact with σ1. These studies could explain the previously described role for μ1 in influencing reovirus pathogenesis. These studies are also of broader significance because they highlight an example of how genetic reassortment between virus strains could produce phenotypes that are distinct from those of either parent.

Viruses and the Diversity of Cell Death

Cell death is a common outcome of virus infection. In some cases, cell death curbs virus replication. In others, cell death enhances virus dissemination and contributes to tissue injury, exacerbating viral disease. Three forms of cell death are observed following virus infection-apoptosis, necroptosis, and pyroptosis. In this review, I describe the core machinery needed for each of these forms of cell death. Using representative viruses, I highlight how distinct stages of virus replication initiate signaling pathways that elicit these forms of cell death. I also discuss viral strategies to overcome the deleterious effects of cell death on virus propagation and the consequences of cell death for host physiology.

Engineering Recombinant Reoviruses To Display gp41 Membrane-Proximal External-Region Epitopes from HIV-1

Vaccines to protect against HIV-1, the causative agent of AIDS, are not approved for use. Antibodies that neutralize genetically diverse strains of HIV-1 bind to discrete regions of the envelope glycoproteins, including the gp41 MPER. We engineered recombinant reoviruses that displayed MPER epitopes in attachment protein σ1 (REO-MPER vectors). The REO-MPER vectors replicated with wild-type efficiency, were genetically stable, and retained native antigenicity. However, we did not detect HIV-1-specific immune responses following inoculation of the REO-MPER vectors into small animals. This work provides proof of principle for engineering reovirus to express antigenic epitopes and illustrates the difficulty in eliciting MPER-specific immune responses.

The gp41 membrane-proximal external region (MPER) is a target for broadly neutralizing antibody responses against human immunodeficiency virus type 1 (HIV-1). However, replication-defective virus vaccines currently under evaluation in clinical trials do not efficiently elicit MPER-specific antibodies. Structural modeling suggests that the MPER forms an α-helical coiled coil that is required for function and immunogenicity. To maintain the native MPER conformation, we used reverse genetics to engineer replication-competent reovirus vectors that displayed MPER sequences in the α-helical coiled-coil tail domain of viral attachment protein σ1. Sequences in reovirus strain type 1 Lang (T1L) σ1 were exchanged with sequences encoding HIV-1 strain Ba-L MPER epitope 2F5 or the entire MPER. Individual 2F5 or MPER substitutions were introduced at virion-proximal or virion-distal sites in the σ1 tail. Recombinant reoviruses containing heterologous HIV-1 sequences were viable and produced progeny yields comparable to those with wild-type virus. HIV-1 sequences were retained following 10 serial passages in cell culture, indicating that the substitutions were genetically stable. Recombinant viruses engineered to display the 2F5 epitope or full-length MPER in σ1 were recognized by purified 2F5 antibody. Inoculation of mice with 2F5-containing vectors or rabbits with 2F5- or MPER-containing vectors elicited anti-reovirus antibodies, but HIV-1-specific antibodies were not detected. Together, these findings indicate that heterologous sequences that form α-helices can functionally replace native sequences in the α-helical tail domain of reovirus attachment protein σ1. However, although these vectors retain native antigenicity, they were not immunogenic, illustrating the difficulty of experimentally inducing immune responses to this essential region of HIV-1.

IMPORTANCE Vaccines to protect against HIV-1, the causative agent of AIDS, are not approved for use. Antibodies that neutralize genetically diverse strains of HIV-1 bind to discrete regions of the envelope glycoproteins, including the gp41 MPER. We engineered recombinant reoviruses that displayed MPER epitopes in attachment protein σ1 (REO-MPER vectors). The REO-MPER vectors replicated with wild-type efficiency, were genetically stable, and retained native antigenicity. However, we did not detect HIV-1-specific immune responses following inoculation of the REO-MPER vectors into small animals. This work provides proof of principle for engineering reovirus to express antigenic epitopes and illustrates the difficulty in eliciting MPER-specific immune responses.

Exploring Reovirus Plasticity for Improving Its Use as Oncolytic Virus

Reoviruses are non-enveloped viruses with a segmented double stranded RNA genome. In humans, they are not associated with serious disease. Human reoviruses exhibit an inherent preference to replicate in tumor cells, which makes them ideally suited for use in oncolytic virotherapies. Their use as anti-cancer agent has been evaluated in several clinical trials, which revealed that intra-tumoral and systemic delivery of reoviruses are well tolerated. Despite evidence of anti-tumor effects, the efficacy of reovirus in anti-cancer monotherapy needs to be further enhanced. The opportunity to treat both the primary tumor as well as metastases makes systemic delivery a preferred administration route. Several pre-clinical studies have been conducted to address the various hurdles connected to systemic delivery of reoviruses. The majority of those studies have been done in tumor-bearing immune-deficient murine models. This thwarts studies on the impact of the contribution of the immune system to the tumor cell eradication. This review focuses on key aspects of the reovirus/host-cell interactions and the methods that are available to modify the virus to alter these interactions. These aspects are discussed with a focus on improving the reovirus' antitumor efficacy.

Viral gene expression potentiates reovirus-induced necrosis

Infection of some cell types by reovirus evokes a caspase-independent form of cell death resembling necrosis. While reovirus strain T3D induces necrosis much more efficiently than strain T1L, which viral components contribute to this difference is not known. In this study, we identified that the sialic acid binding property of the reovirus σ1 protein affects necrosis efficiency. We found that in addition to sialic acid engagement by the virus particles, viral gene expression, in the form of viral RNA or protein synthesis, is also required for necrosis induction. Our studies reveal that sialic acid does not directly participate in necrosis induction by initiating a signaling pathway. Instead, sialic acid engagement augments necrosis induction indirectly, by increasing reovirus gene expression in each infected cell. Comparison of our results with previous studies suggests that reovirus-induced apoptosis and necrosis are initiated by distinct stages of viral infection.

Screening compounds of Chinese medicinal herbs anti-Marek's disease virus

CONTEXT

Marek's disease (MD) seriously threatens the world poultry industry and has resulted in great economic losses. Chinese medicinal herbs are a rich source for lead compounds and drug candidates for antiviral treatments.

OBJECTIVE

To investigate the anti-MDV activity and mechanism of 20 compounds extracted from Chinese medicinal herbs.

MATERIALS AND METHODS

Antiviral assay, time of addition experiments, and virucidal assay were performed on chicken embryo fibroblast cells. The 50% cytotoxic concentration and 50% effective concentration were determined and, accordingly, selectivity index and inhibition ratio were calculated.

RESULTS

Antiviral assay showed dipotassium glycyrrhizinate (DG) and sodium tanshinone IIA sulfonate (STS) exhibited significantly inhibitory activity against MDV in a dose-dependent manner. EC50 of DG and STS were 893.5 ± 36.99 µg/mL and 54.82 ± 2.99 µg/mL, and selective index (SI) were >3.36 and >9.12, respectively. Time of addition experiment and virucidal assay demonstrated DG inhibited viral replication in the full replication cycle and inactivated MDV particles in non-time-dependent manner, but STS interfered with the early stage of MDV replication and inactivated MDV particles in a time-dependent manner. Moreover, both DG and STS promoted apoptosis of cells infected by MDV.

DISCUSSION AND CONCLUSION

DG and STS have great potential for developing new anti-MDV drugs for clinic application.

Multi-organ lesions in suckling mice infected with SARS-associated mammalian reovirus linked with apoptosis induced by viral proteins μ1 and σ1

We reported the isolation and characterization of a novel mammalian reassortant reovirus BYD1 that may have played an accomplice role with SARS-coronavirus during the 2003 SARS pandemic. The pathogenic mechanism of this novel reovirus is unknown. Reovirus pathogenicity has been associated with virus-induced apoptosis in cultured cells and in vivo. The reovirus outer capsid protein μ1 is recognized as the primary determinant of reovirus-induced apoptosis. Here, we investigated the apoptosis induced by BYD1, its outer capsid protein μ1, and its cell-attachment protein σ1 to understand the pathogenesis of BYD1. We also investigated BYD1 caused systemic complications in suckling mice. Under electron microscopy, BYD1-infected cells showed characteristics typical of apoptosis. Notably, ectopically expressed μ1 and σ1 induced similar pathological apoptosis, independent of BYD1 infection, in host cells in which they were expressed, which suggests that μ1 and σ1 are both apoptotic virulence factors. Consistent with previous reports of reovirus pathogenicity, suckling mice intracranially inoculated with BYD1 developed central nerve damage, myocarditis, and pneumonia. Collectively, our data suggest that BYD1 μ1- and σ1-induced apoptosis is involved in the multi-organ lesions in a suckling mouse BYD1 infection model.

The interplay of reovirus with autophagy

Autophagy participates in multiple fundamental physiological processes, including survival, differentiation, development, and cellular homeostasis. It eliminates cytoplasmic protein aggregates and damaged organelles by triggering a series of events: sequestering the protein substrates into double-membrane vesicles, fusing the vesicles with lysosomes, and then degrading the autophagic contents. This degradation pathway is also involved in various disorders, for instance, cancers and infectious diseases. This paper provides an overview of modulation of autophagy in the course of reovirus infection and also the interplay of autophagy and reovirus.

The μ1 72-96 loop controls conformational transitions during reovirus cell entry

The reovirus outer capsid protein μ1 forms a lattice surrounding the viral core. In the native state, μ1 determines the environmental stability of the viral capsid. Additionally, during cell entry, μ1 undergoes structural rearrangements that facilitate delivery of the viral cores across the membrane. To determine how the capsid-stabilizing functions of μ1 impinge on the capacity of μ1 to undergo conformational changes required for cell entry, we characterized viruses with mutations engineered at charged residues within the μ1 loop formed by residues 72 to 96 (72-96 loop). This loop is proposed to stabilize the capsid by mediating interactions between neighboring μ1 trimers and between trimers and the core. We found that mutations at Glu89 (E89) within this loop produced viruses with compromised efficiency for completing their replication cycle. ISVPs of E89 mutants converted to ISVP*s more readily than those of wild-type viruses. The E89 mutants yielded revertants with second-site substitutions within regions that mediate interaction between μ1 trimers at a site distinct from the 72-96 loop. These viruses also contained changes in regions that control interactions within μ1 trimers. Viruses containing these second-site changes displayed restored plaque phenotypes and were capable of undergoing ISVP-to-ISVP* conversion in a regulated manner. These findings highlight regions of μ1 that stabilize the reovirus capsid and demonstrate that an enhanced propensity to form ISVP*s in an unregulated manner compromises viral fitness.

Epizootic hemorrhagic disease virus induces and benefits from cell stress, autophagy, and apoptosis

The mode and timing of virally induced cell death hold the potential of regulating viral yield, viral transmission, and the severity of virally induced disease. Orbiviruses such as the epizootic hemorrhagic disease virus (EHDV) are nonenveloped and cytolytic. To date, the death of cells infected with EHDV, the signal transduction pathways involved in this process, and the consequence of their inhibition have yet to be characterized. Here, we report that the Ibaraki strain of EHDV2 (EHDV2-IBA) induces apoptosis, autophagy, a decrease in cellular protein synthesis, the activation of c-Jun N-terminal kinase (JNK), and the phosphorylation of the JNK substrate c-Jun. The production of infectious virions decreased upon inhibition of apoptosis with the pan-caspase inhibitor Q-VD-OPH (quinolyl-valyl-O-methylaspartyl-[-2,6-difluorophenoxy]-methyl ketone), upon inhibition of autophagy with 3-methyladenine or via the knockout of the autophagy regulator Atg5, or upon treatment of infected cells with the JNK inhibitor SP600125 or the cyclin-dependent kinase (CDK) inhibitor roscovitine, which also inhibited c-Jun phosphorylation. Moreover, Q-VD-OPH, SP600125, and roscovitine partially reduced EHDV2-IBA-induced cell death, and roscovitine diminished the induction of autophagy by EHDV2-IBA. Taken together, our results imply that EHDV induces and benefits from the activation of signaling pathways involved in cell stress and death.

Nonstructural protein σ1s mediates reovirus-induced cell cycle arrest and apoptosis

Reovirus nonstructural protein σ1s is implicated in cell cycle arrest at the G2/M boundary and induction of apoptosis. However, the contribution of σ1s to these effects in an otherwise isogenic viral background has not been defined. To evaluate the role of σ1s in cell cycle arrest and apoptosis, we used reverse genetics to generate a σ1s-null reovirus. Following infection with wild-type virus, we observed an increase in the percentage of cells in G2/M, whereas the proportion of cells in G2/M following infection with the σ1s-null mutant was unaffected. Similarly, we found that the wild-type virus induced substantially greater levels of apoptosis than the σ1s-null mutant. These data indicate that σ1s is required for both reovirus-induced cell cycle arrest and apoptosis. To define sequences in σ1s that mediate these effects, we engineered viruses encoding C-terminal σ1s truncations by introducing stop codons in the σ1s open reading frame. We also generated viruses in which charged residues near the σ1s amino terminus were replaced individually or as a cluster with nonpolar residues. Analysis of these mutants revealed that amino acids 1 to 59 and the amino-terminal basic cluster are required for induction of both cell cycle arrest and apoptosis. Remarkably, viruses that fail to induce cell cycle arrest and apoptosis also are attenuated in vivo. Thus, identical sequences in σ1s are required for reovirus-induced cell cycle arrest, apoptosis, and pathogenesis. Collectively, these findings provide evidence that the σ1s-mediated properties are genetically linked and suggest that these effects are mechanistically related.

Apoptosis induction influences reovirus replication and virulence in newborn mice

Apoptosis is a type of controlled cell death that is essential for development and tissue homeostasis. It also serves as a robust host response against infection by many viruses. The capacity of neurotropic viruses to induce apoptosis strongly correlates with virulence. However, the precise function of apoptosis in viral infection is not well understood. Reovirus is a neurotropic virus that induces apoptosis in a variety of cell types, including central nervous system neurons, leading to fatal encephalitis in newborn mice. To determine the effect of apoptosis on reovirus replication in the host, we generated two otherwise isogenic viruses that differ in a single amino acid in viral capsid protein μ1 that segregates with apoptotic capacity. Apoptosis-proficient and apoptosis-deficient viruses were compared for replication, dissemination, tropism, and tissue injury in newborn mice and for the capacity to spread to uninfected littermates. Our results indicate that apoptotic capacity enhances reovirus replication in the brain and consequent neurovirulence but reduces transmission efficiency. The replication advantage of the apoptosis-proficient strain is limited to the brain and correlates with enhanced infectivity of neurons. These studies reveal a new cell type-specific determinant of reovirus virulence.

Reovirus activates a caspase-independent cell death pathway

Virus-induced apoptosis is thought to be the primary mechanism of cell death following reovirus infection. Induction of cell death following reovirus infection is initiated by the incoming viral capsid proteins during cell entry and occurs via NF-κB-dependent activation of classical apoptotic pathways. Prototype reovirus strain T3D displays a higher cell-killing potential than strain T1L. To investigate how signaling pathways initiated by T3D and T1L differ, we methodically analyzed cell death pathways activated by these two viruses in L929 cells. We found that T3D activates NF-κB, initiator caspases, and effector caspases to a significantly greater extent than T1L. Surprisingly, blockade of NF-κB or caspases did not affect T3D-induced cell death. Cell death following T3D infection resulted in a reduction in cellular ATP levels and was sensitive to inhibition of the kinase activity of receptor interacting protein 1 (RIP1). Furthermore, membranes of T3D-infected cells were compromised. Based on the dispensability of caspases, a requirement for RIP1 kinase function, and the physiological status of infected cells, we conclude that reovirus can also induce an alternate, necrotic form of cell death described as necroptosis. We also found that induction of necroptosis requires synthesis of viral RNA or proteins, a step distinct from that necessary for the induction of apoptosis. Thus, our studies reveal that two different events in the reovirus replication cycle can injure host cells by distinct mechanisms.

Virus-induced cell death is a determinant of pathogenesis. Mammalian reovirus is a versatile experimental model for identifying viral and host intermediaries that contribute to cell death and for examining how these factors influence viral disease. In this study, we identified that in addition to apoptosis, a regulated form of cell death, reovirus is capable of inducing an alternate form of controlled cell death known as necroptosis. Death by this pathway perturbs the integrity of host membranes and likely triggers inflammation. We also found that apoptosis and necroptosis following viral infection are activated by distinct mechanisms. Our results suggest that host cells can detect different stages of viral infection and attempt to limit viral replication through different forms of cellular suicide. While these death responses may aid in curbing viral spread, they can also exacerbate tissue injury and disease following infection.

Reovirus receptors, cell entry, and proapoptotic signaling

Mammalian orthoreoviruses (reoviruses) are members of the Reoviridae. Reoviruses contain 10 double-stranded (ds) RNA gene segments enclosed in two concentric protein shells, called outer capsid and core. These viruses serve as a versatile experimental system for studies of viral replication events at the virus-cell interface, including engagement of cell-surface receptors, internalization and disassembly, and activation of the innate immune response, including NF-κB-dependent cellular signaling pathways. Reoviruses also provide a model system for studies of virus-induced apoptosis and organ-specific disease in vivo.Reoviruses attach to host cells via the filamentous attachment protein, σ1. The σ1 protein of all reovirus serotypes engages junctional adhesion molecule-A (JAM-A), an integral component of intercellular tight junctions. The σ1 protein also binds to cell-surface carbohydrate, with the type of carbohydrate bound varying by serotype. Following attachment to JAM-A and carbohydrate, reovirus internalization is mediated by β1 integrins, most likely via clathrin-dependent endocytosis. In the endocytic compartment, reovirus outer-capsid protein σ3 is removed by acid-dependent cysteine proteases in most cell types. Removal of σ3 results in the exposure of a hydrophobic conformer of the viral membrane-penetration protein, μ1, which pierces the endosomal membrane and delivers transcriptionally active reovirus core particles into the cytoplasm.Reoviruses induce apoptosis in both cultured cells and infected mice. Perturbation of reovirus disassembly using inhibitors of endosomal acidification or protease activity abrogates apoptosis. The μ1-encoding M2 gene is genetically linked to strain-specific differences in apoptosis-inducing capacity, suggesting a function for μ1 in induction of death signaling. Reovirus disassembly leads to activation of transcription factor NF-κB, which modulates apoptotic signaling in numerous types of cells. Inhibition of NF-κB nuclear translocation using either pharmacologic agents or expression of transdominant forms of IκB blocks reovirus-induced apoptosis, suggesting an essential role for NF-κB activation in the death response. Multiple effector pathway s downstream of NF-κB-directed gene expression execute reovirus-induced cell death. This chapter will focus on the mechanisms by which reovirus attachment and disassembly activate NF-κB and stimulate the cellular proapoptotic machinery.

Utilization of sialylated glycans as coreceptors enhances the neurovirulence of serotype 3 reovirus

Mammalian reoviruses display serotype-specific patterns of tropism and disease in the murine central nervous system (CNS) attributable to polymorphisms in viral attachment protein σ1. While all reovirus serotypes use junctional adhesion molecule-A as a cellular receptor, they differ in their utilization of carbohydrate coreceptors. This observation raises the possibility that carbohydrate binding by σ1 influences reovirus pathology in the CNS. In this study, we sought to define the function of carbohydrate binding in reovirus neuropathogenesis. Newborn mice were inoculated intramuscularly with wild-type strain type 3 Dearing (T3D) and T3D-σ1R202W, a point mutant T3D derivative that does not bind sialic acid (SA). Infected mice were monitored for survival, and viral loads at the sites of primary and secondary replication were quantified. Fewer mice inoculated with the wild-type virus survived in comparison to those inoculated with the mutant virus. The wild-type virus also produced higher titers in the spinal cord and brain at late times postinoculation but lower titers in the liver in comparison to those produced by the mutant virus. In addition, the wild-type virus was more virulent and produced higher titers in the brain than the mutant following intracranial inoculation. These animal infectivity studies suggest that T3D-σ1R202W harbors a defect in neural growth. Concordantly, compared with the wild-type virus, the mutant virus displayed a decreased capacity to infect and replicate in primary cultures of cortical neurons, a property dependent on cell surface SA. These results suggest that SA binding enhances the kinetics of reovirus replication in neural tissues and highlight a functional role for sialylated glycans as reovirus coreceptors in the CNS.

Transport to late endosomes is required for efficient reovirus infection

Rab GTPases play an essential role in vesicular transport by coordinating the movement of various types of cargo from one cellular compartment to another. Individual Rab GTPases are distributed to specific organelles and thus serve as markers for discrete types of endocytic vesicles. Mammalian reovirus binds to cell surface glycans and junctional adhesion molecule-A (JAM-A) and enters cells by receptor-mediated endocytosis in a process dependent on β1 integrin. Within organelles of the endocytic compartment, reovirus undergoes stepwise disassembly catalyzed by cathepsin proteases, which allows the disassembly intermediate to penetrate endosomal membranes and release the transcriptionally active viral core into the cytoplasm. The pathway used by reovirus to traverse the endocytic compartment is largely unknown. In this study, we found that reovirus particles traffic through early, late, and recycling endosomes during cell entry. After attachment to the cell surface, reovirus particles and JAM-A codistribute into each of these compartments. Transfection of cells with constitutively active and dominant-negative Rab GTPases that affect early and late endosome biogenesis and maturation influenced reovirus infectivity. In contrast, reovirus infectivity was not altered in cells expressing mutant Rab GTPases that affect recycling endosomes. Thus, reovirus virions localize to early, late, and recycling endosomes during entry into host cells, but only those that traverse early and late endosomes yield a productive infection.

Cell entry-associated conformational changes in reovirus particles are controlled by host protease activity

Membrane penetration by reovirus requires successive formation of two cell entry intermediates, infectious subvirion particles (ISVPs) and ISVP*s. In vitro incubation of reovirus virions with high concentration of chymotrypsin (CHT) results in partial digestion of the viral outer capsid to form ISVPs. When virions are instead digested with low concentrations of chymotrypsin, the outer capsid is completely proteolyzed to form cores. We investigated the basis for the inverse relationship between CHT activity and protease susceptibility of the reovirus outer capsid. We report that core formation following low-concentration CHT digestion proceeds via formation of particles that contain a protease-sensitive form of the μ1C protein, a characteristic of ISVP*s. In addition, we found that both biochemical features and viral genetic requirements for ISVP* formation and core formation following low-concentration CHT digestion are identical, suggesting that core formation proceeds via a particle resembling ISVP*s. Furthermore, we determined that intermediates generated following low-concentration CHT digestion are distinct from ISVPs and convert to ISVP*-like particles much more readily than ISVPs. These results suggest that the activity of host proteases used to generate ISVPs can influence the efficiency with which the next step in reovirus cell entry, namely, ISVP-to-ISVP* conversion, occurs.

Addition of exogenous polypeptides on the mammalian reovirus outer capsid using reverse genetics

Addition of exogenous peptide sequences on viral capsids is a powerful approach to study the process of viral infection or to retarget viruses toward defined cell types. Until recently, it was not possible to manipulate the genome of mammalian reovirus and this was an obstacle to the addition of exogenous sequence tags onto the capsid of a replicating virus. This obstacle has now been overcome by the availability of the plasmid-based reverse genetics system. In the present study, reverse genetics was used to introduce different exogenous peptides, up to 40 amino acids long, at the carboxyl-terminal end of the σ1 outer capsid protein. The tagged viruses obtained were infectious, produce plaques of similar size, and could be easily propagated at high titers. However, attempts to introduce a 750 nucleotides-long sequence failed, even when it was added after the stop codon, suggesting a possible size limitation at the nucleic acid level.

Apoptosis induced by mammalian reovirus is beta interferon (IFN) independent and enhanced by IFN regulatory factor 3- and NF-κB-dependent expression of Noxa

A variety of signal transduction pathways are activated in response to viral infection, which dampen viral replication and transmission. These mechanisms involve both the induction of type I interferons (IFNs), which evoke an antiviral state, and the triggering of apoptosis. Mammalian orthoreoviruses are double-stranded RNA viruses that elicit apoptosis in vitro and in vivo. The transcription factors interferon regulatory factor 3 (IRF-3) and nuclear factor kappa light-chain enhancer of activated B cells (NF-κB) are required for the expression of IFN-β and the efficient induction of apoptosis in reovirus-infected cells. However, it is not known whether IFN-β induction is required for apoptosis, nor have the genes induced by IRF-3 and NF-κB that are responsible for apoptosis been identified. To determine whether IFN-β is required for reovirus-induced apoptosis, we used type I IFN receptor-deficient cells, IFN-specific antibodies, and recombinant IFN-β. We found that IFN synthesis and signaling are dispensable for the apoptosis of reovirus-infected cells. These results indicate that the apoptotic response following reovirus infection is mediated directly by genes responsive to IRF-3 and NF-κB. Noxa is a proapoptotic BH3-domain-only protein of the Bcl-2 family that requires IRF-3 and NF-κB for efficient expression. We found that Noxa is strongly induced at late times (36 to 48 h) following reovirus infection in a manner dependent on IRF-3 and NF-κB. The level of apoptosis induced by reovirus is significantly diminished in cells lacking Noxa, indicating a key prodeath function for this molecule during reovirus infection. These results suggest that prolonged innate immune response signaling induces apoptosis by eliciting Noxa expression in reovirus-infected cells.

Adenovirus type 5 rupture of lysosomes leads to cathepsin B-dependent mitochondrial stress and production of reactive oxygen species

In response to viral infection, reactive oxygen species (ROS) mediate innate immune signaling or generate danger signals to activate immune cells. The mechanisms of virally induced ROS are poorly defined, however. We demonstrate that ROS are produced within minutes of adenovirus type 5 (Ad5) infection of macrophages and that oxidative stress supports Ad5-induced cytokine secretion. We show that short hairpin RNA (shRNA) knockdown of TLR9 has no effect on ROS production despite observed decreases in Ad-induced cytokine secretion. A major source of ROS in macrophages is NADPH oxidase. However, shRNA knockdown of the NADPH oxidase subunit NOX2 does not attenuate Ad-induced ROS. Induction of ROS is not observed in cells infected with a temperature-sensitive mutant of Ad2, ts1, which is defective in endosomal membrane penetration during cell entry. Further, Ad5, but not ts1, induces the release of lysosomal cathepsin B into the cytoplasm of infected cells. In agreement with this finding, we observe a loss of mitochondrial membrane potential upon Ad infection which requires Ad endosomal membrane penetration and cathepsin B activity. Overexpression of Bcl-2 attenuates Ad5-induced ROS, further supporting the role for mitochondrial membrane destabilization as the source of ROS in response to Ad5 infection. Together, these data suggest that ROS produced in response to Ad5 infection depends on the virally induced endosomal membrane rupture to release lysosomal cathepsins. Furthermore, the release of cathepsins leads to mitochondrial membrane disruption and thus the release of ROS from the mitochondria.

Reverse genetics for mammalian reovirus

Mammalian orthoreoviruses (reoviruses) are highly tractable models for studies of viral replication and pathogenesis. The versatility of reovirus as an experimental model has been enhanced by development of a plasmid-based reverse genetics system. Infectious reovirus can be recovered from cells transfected with plasmids encoding cDNAs of each reovirus gene segment using a strategy that does not require helper virus and is independent of selection. In this system, transcription of each gene segment is driven by bacteriophage T7 RNA polymerase, which can be supplied transiently by recombinant vaccinia virus (rDIs-T7pol) or by cells that constitutively express the enzyme. Reverse genetics systems have been developed for two prototype reovirus strains, type 1 Lang (T1L) and type 3 Dearing (T3D). Each reovirus cDNA was encoded on an independent plasmid for the first-generation rescue system. The efficiency of virus recovery was enhanced in a second-generation system by combining the cDNAs for multiple reovirus gene segments onto single plasmids to reduce the number of plasmids from 10 to 4. The reduction in plasmid number and the use of baby hamster kidney cells that express T7 RNA polymerase increased the efficiency of viral rescue, reduced the incubation time required to recover infectious virus, and eliminated potential biosafety concerns associated with the use of recombinant vaccinia virus. Reovirus reverse genetics has been used to introduce mutations into viral capsid and nonstructural components to study viral protein-structure activity relationships and can be exploited to engineer recombinant reoviruses for vaccine and oncolytic applications.

Lysosomal localization and mechanism of membrane penetration influence nonenveloped virus activation of the NLRP3 inflammasome

Adenovirus (Ad) endosomal membrane penetration activates the NLRP3 inflammasome by releasing lysosomal cathepsin B (catB) into the cytoplasm. We therefore examined the extent to which inflammasome activation correlates with Ad colocalization with catB-enriched lysosomes. Inflammasome activation, is greater during infections with Ad5 possessing an Ad16 fiber (Ad5F16gfp), or Ad5gfp neutralized by human serum, than Ad5gfp alone. Enhanced IL-1β release by Ad5F16gfp is partially due to increased TLR9 signaling but also correlates with greater release of catB into the cytoplasm. This increased TLR9 signaling and catB release correlates with a greater localization of Ad5F16gfp to lysosomes prior to endosomal escape. Another nonenveloped virus, reovirus, requires catB to penetrate cell membranes. However, reovirus did not release catB into the cytoplasm despite significantly greater colocalization with lysosomes compared to Ad5gfp and efficient membrane penetration. Thus, not only lysosomal localization, but the mechanism of membrane penetration influences viral activation of the NLRP3 inflammasome.

Enter the kill zone: initiation of death signaling during virus entry

Infection of host cells by a variety of viruses results in programmed cell death or apoptosis. In many cases, early events in virus replication that occur prior to synthesis of viral proteins and replication of viral genomes directly or indirectly activate signaling pathways that culminate in cell death. Using examples of viruses for which prodeath signaling is better defined, this review will describe how cell entry steps including virus attachment to receptors, virus uncoating in endosomes, and events that occur following membrane penetration lead to apoptosis. The relevance and physiologic consequences of early induction of prodeath signaling to viral pathogenesis also will be discussed.

From touchdown to transcription: the reovirus cell entry pathway

Mammalian orthoreoviruses (reoviruses) are prototype members of the Reoviridae family of nonenveloped viruses. Reoviruses contain ten double-stranded RNA gene segments enclosed in two concentric protein shells, outer capsid and core. These viruses serve as a versatile experimental system for studies of virus cell entry, innate immunity, and organ-specific disease. Reoviruses engage cells by binding to cell-surface carbohydrates and the immunoglobulin superfamily member, junctional adhesion molecule-A (JAM-A). JAM-A is a homodimer formed by extensive contacts between its N-terminal immunoglobulin-like domains. Reovirus attachment protein σ1 disrupts the JAM-A dimer, engaging a single JAM-A molecule by virtually the same interface used for JAM-A homodimerization. Following attachment to JAM-A and carbohydrate, reovirus internalization is promoted by β1 integrins, most likely via clathrin-dependent endocytosis. In the endocytic compartment, reovirus outer-capsid protein σ3 is removed by cathepsin proteases, which exposes the viral membrane-penetration protein, μ1. Proteolytic processing and conformational rearrangements of μ1 mediate endosomal membrane rupture and delivery of transcriptionally active reovirus core particles into the host cell cytoplasm. These events also allow the φ cleavage fragment of μ1 to escape into the cytoplasm where it activates NF-κB and elicits apoptosis. This review will focus on mechanisms of reovirus cell entry and activation of innate immune response signaling pathways.

Determinants of strain-specific differences in efficiency of reovirus entry

Cell entry of reovirus requires a series of ordered steps, which include conformational changes in outer capsid protein μ1 and its autocleavage. The μ1N fragment released as a consequence of these events interacts with host cell membranes and mediates their disruption, leading to delivery of the viral core into the cytoplasm. The prototype reovirus strains T1L and T3D exhibit differences in the efficiency of autocleavage, in the propensity to undergo conformational changes required for membrane penetration, and in the capacity for penetrating host cell membranes. To better understand how polymorphic differences in μ1 influence reovirus entry events, we generated recombinant viruses that express chimeric T1L-T3D μ1 proteins and characterized them for the capacity to efficiently complete each step required for membrane penetration. Our studies revealed two important functions for the central δ region of μ1. First, we found that μ1 autocleavage is regulated by the N-terminal portion of δ, which forms an α-helical pedestal structure. Second, we observed that the C-terminal portion of δ, which forms a jelly-roll β barrel structure, regulates membrane penetration by influencing the efficiency of ISVP* formation. Thus, our studies highlight the molecular basis for differences in the membrane penetration efficiency displayed by prototype reovirus strains and suggest that distinct portions of the reovirus δ domain influence different steps during entry.

A proapoptotic peptide derived from reovirus outer capsid protein {micro}1 has membrane-destabilizing activity

The reovirus outer capsid protein μ1 is responsible for cell membrane penetration during virus entry and contains determinants necessary for virus-induced apoptosis. Residues 582 to 611 of μ1 are necessary and sufficient for reovirus-induced apoptosis, and residues 594 and 595 independently regulate the efficiency of viral entry and reovirus-induced cell apoptosis, respectively. Two of three α-helices within this region, helix 1 (residues 582 to 611) and helix 3 (residues 644 to 675), play a role in reovirus-induced apoptosis. Here, we chemically synthesized peptides representing helix 1 (H1), H1:K594D, H1:I595K, and helix 3 (H3) and examined their biological properties. We found that H1, but not H3, was able to cause concentration- and size-dependent leakage of molecules from small unilamellar liposomes. We further found that direct application of H1, but not H1:K594D, H1:I595K, or H3, to cells resulted in cytotoxicity. Application of the H1 peptide to L929 cells caused rapid elevations in intracellular calcium concentration that were independent of phospholipase C activation. Cytotoxicity of H1 was not restricted to eukaryotic cells, as the H1 peptide also had bactericidal activity. Based on these findings, we propose that the proapoptotic function of the H1 region of μ1 is dependent on its capacity to destabilize cellular membranes and cause release of molecules from intracellular organelles that ultimately induces cell necrosis or apoptosis, depending on the dose.

Reovirus infection or ectopic expression of outer capsid protein micro1 induces apoptosis independently of the cellular proapoptotic proteins Bax and Bak

Mammalian orthoreoviruses induce apoptosis in vivo and in vitro; however, the specific mechanism by which apoptosis is induced is not fully understood. Recent studies have indicated that the reovirus outer capsid protein μ1 is the primary determinant of reovirus-induced apoptosis. Ectopically expressed μ1 induces apoptosis and localizes to intracellular membranes. Here we report that ectopic expression of μ1 activated both the extrinsic and intrinsic apoptotic pathways with activation of initiator caspases-8 and -9 and downstream effector caspase-3. Activation of both pathways was required for μ1-induced apoptosis, as specific inhibition of either caspase-8 or caspase-9 abolished downstream effector caspase-3 activation. Similar to reovirus infection, ectopic expression of μ1 caused release into the cytosol of cytochrome c and smac/DIABLO from the mitochondrial intermembrane space. Pancaspase inhibitors did not prevent cytochrome c release from cells expressing μ1, indicating that caspases were not required. Additionally, μ1- or reovirus-induced release of cytochrome c occurred efficiently in Bax(-/-)Bak(-/-) mouse embryonic fibroblasts (MEFs). Finally, we found that reovirus-induced apoptosis occurred in Bax(-/-)Bak(-/-) MEFs, indicating that reovirus-induced apoptosis occurs independently of the proapoptotic Bcl-2 family members Bax and Bak.