Reovirus infection and tissue injury in the mouse central nervous system are associated with apoptosis

NgR1 binding to reovirus reveals an unusual bivalent interaction and a new viral attachment protein

Nogo-66 receptor 1 (NgR1) binds a variety of structurally dissimilar ligands in the adult central nervous system to inhibit axon extension. Disruption of ligand binding to NgR1 and subsequent signaling can improve neuron outgrowth, making NgR1 an important therapeutic target for diverse neurological conditions such as spinal crush injuries and Alzheimer's disease. Human NgR1 serves as a receptor for mammalian orthoreovirus (reovirus), but the mechanism of virus-receptor engagement is unknown. To elucidate how NgR1 mediates cell binding and entry of reovirus, we defined the affinity of interaction between virus and receptor, determined the structure of the virus-receptor complex, and identified residues in the receptor required for virus binding and infection. These studies revealed that central NgR1 surfaces form a bridge between two copies of viral capsid protein σ3, establishing that σ3 serves as a receptor ligand for reovirus. This unusual binding interface produces high-avidity interactions between virus and receptor to prime early entry steps. These studies refine models of reovirus cell-attachment and highlight the evolution of viruses to engage multiple receptors using distinct capsid components.

Oncolytic Reoviruses: Can These Emerging Zoonotic Reoviruses Be Tamed and Utilized?

Orthoreovirus is a nonenveloped double-stranded RNA virus under the Reoviridae family. This group of viruses, especially mammalian orthoreovirus (MRV), are reported with great therapeutic values due to their oncolytic effects. In this review, the life cycle and oncolytic effect of MRV and a few emerging reoviruses were summarized. This article also highlights the challenges and strategies of utilizing MRV and the emerging reoviruses, avian orthoreovirus (ARV) and pteropine orthoreovirus (PRV), as oncolytic viruses (OVs). Besides, the emergence of potential ARV and PRV as OVs were discussed in comparison to MRV. Finally, the risk of reovirus as zoonosis or reverse zoonosis (zooanthroponosis) were debated, and concerns were raised in this article, which warrant continue surveillance of reovirus (MRV, ARV, and PRV) in animals, humans, and the environment.

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.

The M2 Gene Is a Determinant of Reovirus-Induced Myocarditis

Although a broad range of viruses cause myocarditis, the mechanisms that underlie viral myocarditis are poorly understood. Here, we report that the M2 gene is a determinant of reovirus myocarditis. The M2 gene encodes outer capsid protein μ1, which mediates host membrane penetration during reovirus entry. We infected newborn C57BL/6 mice with reovirus strain type 1 Lang (T1L) or a reassortant reovirus in which the M2 gene from strain type 3 Dearing (T3D) was substituted into the T1L genetic background (T1L/T3DM2). T1L was non-lethal in wild-type mice, whereas greater than 90% of mice succumbed to T1L/T3DM2 infection. T1L/T3DM2 produced higher viral loads than T1L at the site of inoculation. In secondary organs, T1L/T3DM2 was detected with more rapid kinetics and reached higher peak titers than T1L. We found that hearts from T1L/T3DM2-infected mice were grossly abnormal, with large lesions indicative of substantial inflammatory infiltrate. Lesions in T1L/T3DM2-infected mice contained necrotic cardiomyocytes with pyknotic debris, and extensive lymphocyte and histiocyte infiltration. In contrast, T1L induced the formation of small purulent lesions in a small subset of animals, consistent with T1L being mildly myocarditic. Finally, more activated caspase-3-positive cells were observed in hearts from animals infected with T1L/T3DM2 compared to T1L. Together, our findings indicate that substitution of the T3D M2 allele into an otherwise T1L genetic background is sufficient to change a non-lethal infection into a lethal infection. Our results further indicate that T3D M2 enhances T1L replication and dissemination in vivo, which potentiates the capacity of reovirus to cause myocarditis. IMPORTANCE Reovirus is a non-enveloped virus with a segmented double-stranded RNA genome that serves as a model for studying viral myocarditis. The mechanisms by which reovirus drives myocarditis development are not fully elucidated. We found that substituting the M2 gene from strain type 3 Dearing (T3D) into an otherwise type 1 Lang (T1L) genetic background (T1L/T3DM2) was sufficient to convert the non-lethal T1L strain into a lethal infection in neonatal C57BL/6 mice. T1L/T3DM2 disseminated more efficiently and reached higher maximum titers than T1L in all organs tested, including the heart. T1L is mildly myocarditic and induced small areas of cardiac inflammation in a subset of mice. In contrast, hearts from mice infected with T1L/T3DM2 contained extensive cardiac inflammatory infiltration and more activated caspase-3-positive cells, which is indicative of apoptosis. Together, our findings identify the reovirus M2 gene as a new determinant of reovirus-induced myocarditis.

Colorado tick fever virus induces apoptosis in human endothelial cells to facilitate viral replication

Colorado tick fever virus (CTFV) belongs to the genus Coltivirus of the Reoviridae family, and it is the causative agent of Colorado tick fever. Symptoms of the infection are characterized by sudden biphasic fever, headache, and petechial rash, while severe forms of the disease can include meningoencephalitis, hemorrhagic fever, and death in children. However, the mechanisms underlying CTFV induced pathology and severe complications remain unknown. As CTFV is spread by tick bites and disseminates systemically via hematogenous routes, we performed in vitro analysis examining the interactions between endothelial cells (ECs) and CTFV. Our findings indicate that dermal microvascular ECs, HMEC-1, are susceptible and permissive to CTFV infection. To investigate the role of CTFV infection on endothelial barrier function, we assessed transendothelial electrical resistance (TEER) by xCELLigence and observed a dose-dependent decrease in cell index, indicating increased vascular permeability starting at approximately hour 18 (MOI=1) and hour 26 (MOI=0.1). Since CTFV induced cytopathic effect and increased vascular permeability in HMEC-1 cells, we hypothesized that CTFV causes apoptotic cell death. Our results showed that HMEC-1 cells infected with CTFV at 48 h caused a significant increase in Annexin V staining with reduced viability compared to uninfected cells suggesting CTFV induces apoptotic cell death in human ECs. Electron microscopy also was consistent with apoptotic features, including chromatin condensation and cell blebbing. Furthermore, CTFV induced caspase-3/7 activation at 24 and 48 h post-infection (hpi). The inhibition of caspase activity using Z-VAD-FMK reduced CTFV induced cell death and significantly reduced viral titer. These results indicated that CTFV can infect ECs, exerting direct adverse effects, leading to vascular permeability and cell death. Overall, our data suggest that caspase-mediated apoptosis is a critical mechanism by which CTFV induces disease in the host and enhances viral replication. Future studies will examine the viral and cellular determinants involved in CTFV induced apoptosis in human ECs.

Development of a reverse transcription loop mediated isothermal amplification assay for the detection of Mouse reovirus type 3 in laboratory mice

Mouse reovirus type 3 (Reo-3) infection is a viral disease that is harmful for laboratory mice. No rapid and accurate detection methods are currently available for this infection. In this study, we describe a rapid, simple, closed-tube, one step, reverse transcription-loop-mediated isothermal amplification (RT-LAMP) assay for Reo-3 and compare our assay with indirect enzyme-linked immunosorbent assay (ELISA). Three sets of RT-LAMP primers were designed by sequence analysis of a specific conserved sequence of the Reo-3 S1 gene. Using RS2 primer set, the RT-LAMP assay required 60 min at 65 °C to amplify the S1 gene in one step by using Reo-3 RNA template and had no cross-reactivity with the other related pathogens, such as Sendai virus (SV), pneumonia virus of mice (PVM), mouse hepatitis virus (MHV), Ectromelia virus (Ect), minute virus of mice (MVM), P. pneumotropica, B. bronchiseptica, K. pneumonia and P. aeruginosa. in our LAMP reaction system. The limit of detection (LOD) of our RT-LAMP assay is 4 fg/μL. The established RT-LAMP assay enabled visual detection when fluorescence detection reagents were added, and was demonstrated to be effective and efficient. We tested 30 clinical blood samples and five artificial positive samples from SPF mice, the concordance between the two methods for blood samples was 100% compared with indirect ELISA and RT-PCR. Considering its performance, specificity, sensitivity, and repeatability, the developed RT-LAMP could be a valuable tool to supply a more effective Reo-3 detection method in laboratory animal quality monitoring.

Cytidine Monophosphate N-Acetylneuraminic Acid Synthetase and Solute Carrier Family 35 Member A1 Are Required for Reovirus Binding and Infection

Engagement of cell surface receptors by viruses is a critical determinant of viral tropism and disease. The reovirus attachment protein σ1 binds sialylated glycans and proteinaceous receptors to mediate infection, but the specific requirements for different cell types are not entirely known. To identify host factors required for reovirus-induced cell death, we conducted a CRISPR-knockout screen targeting over 20,000 genes in murine microglial BV2 cells. Candidate genes required for reovirus to cause cell death were highly enriched for sialic acid synthesis and transport. Two of the top candidates identified, CMP N-acetylneuraminic acid synthetase (Cmas) and solute carrier family 35 member A1 (Slc35a1), promote sialic acid expression on the cell surface. Two reovirus strains that differ in the capacity to bind sialic acid, T3SA⁺ and T3SA^-, were used to evaluate Cmas and Slc35a1 as potential host genes required for reovirus infection. Following CRISPR-Cas9 disruption of either gene, cell surface expression of sialic acid was diminished. These results correlated with decreased binding of strain T3SA⁺, which is capable of engaging sialic acid. Disruption of either gene did not alter the low-level binding of T3SA^-, which does not engage sialic acid. Furthermore, infectivity of T3SA⁺ was diminished to levels similar to those of T3SA^- in cells lacking Cmas and Slc35a1 by CRISPR ablation. However, exogenous expression of Cmas and Slc35a1 into the respective null cells restored sialic acid expression and T3SA⁺ binding and infectivity. These results demonstrate that Cmas and Slc35a1, which mediate cell surface expression of sialic acid, are required in murine microglial cells for efficient reovirus binding and infection.IMPORTANCE Attachment factors and receptors are important determinants of dissemination and tropism during reovirus-induced disease. In a CRISPR cell survival screen, we discovered two genes, Cmas and Slc35a1, which encode proteins required for sialic acid expression on the cell surface and mediate reovirus infection of microglial cells. This work elucidates host genes that render microglial cells susceptible to reovirus infection and expands current understanding of the receptors on microglial cells that are engaged by reovirus. Such knowledge may lead to new strategies to selectively target microglial cells for oncolytic applications.

Ins and Outs of Reovirus: Vesicular Trafficking in Viral Entry and Egress

Cell entry and egress are essential steps in the viral life cycle that govern pathogenesis and spread. Mammalian orthoreoviruses (reoviruses) are nonenveloped viruses implicated in human disease that serve as tractable models for studies of pathogen-host interactions. In this review we discuss the function of intracellular vesicular transport systems in reovirus entry, trafficking, and egress and comment on shared themes for diverse viruses. Designing strategic therapeutic interventions that impede these steps in viral replication requires a detailed understanding of mechanisms by which viruses coopt vesicular trafficking. We illuminate such targets, which may foster development of antiviral agents.

Neuronal maturation reduces the type I IFN response to orthobunyavirus infection and leads to increased apoptosis of human neurons

BACKGROUND

La Crosse virus (LACV) is the leading cause of pediatric arboviral encephalitis in the USA. LACV encephalitis can result in learning and memory deficits, which may be due to infection and apoptosis of neurons in the brain. Despite neurons being the primary cell infected in the brain by LACV, little is known about neuronal responses to infection.

METHODS

Human cerebral organoids (COs), which contain a spectrum of developing neurons, were used to examine neuronal responses to LACV. Plaque assay and quantitative reverse transcription (qRT) PCR were used to determine the susceptibility of COs to LACV infection. Immunohistochemistry, flow cytometry, and single-cell transcriptomics were used to determine specific neuronal subpopulation responses to the virus.

RESULTS

Overall, LACV readily infected COs causing reduced cell viability and increased apoptosis. However, it was determined that neurons at different stages of development had distinct responses to LACV. Both neural progenitors and committed neurons were infected with LACV, however, committed neurons underwent apoptosis at a higher rate. Transcriptomic analysis showed that committed neurons expressed fewer interferon (IFN)-stimulated genes (ISGs) and genes involved IFN signaling in response to infection compared to neural progenitors. Furthermore, induction of interferon signaling in LACV-infected COs by application of recombinant IFN enhanced cell viability.

CONCLUSIONS

These findings indicate that neuronal maturation increases the susceptibility of neurons to LACV-induced apoptosis. This susceptibility is likely due, at least in part, to mature neurons being less responsive to virus-induced IFN as evidenced by their poor ISG response to LACV. Furthermore, exogenous administration of recombinant IFN to LACV COs rescued cellular viability suggesting that increased IFN signaling is overall protective in this complex neural tissue. Together these findings indicate that induction of IFN signaling in developing neurons is an important deciding factor in virus-induced cell death.

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.

Detection of mammalian orthoreovirus type-3 (Reo-3) infections in mice based on serotype-specific hemagglutination protein sigma-1

Background

Reovirus type-3 infections cause severe pathologies in young mice and thus influence animal experiments in many ways. Therefore, the Federation of Laboratory Animal Science Associations (FELASA) recommends an annual screening in laboratory mice as part of a thorough health monitoring program. Based on the high protein sequence homology among the different reovirus serotypes, immunofluorescence antibody assay and other indirect methods relying on the whole virus are presumably cross-reactive to antibodies triggered by mammalian orthoreovirus infections independent of the serotype.

Methods

The serotype-specific protein σ-1 was expressed in Escherichia coli with an N-terminal Strep-tag and a C-terminal His-tag. The purified Strep-rσ-1-His-construct was used to develop an indirect ELISA by testing defined positive and negative sera obtained by experimental infection of mice as well as field sera.

Results

The Strep-rσ-1-His-ELISA provided high sensitivity and specificity during validation. Notably, a high selectivity was also observed for sera positively tested for other relevant FELASA-listed pathogens. Screening of field samples indicated that a commercial reovirus type-3-based ELISA might be cross-reactive to other murine reovirus serotypes and thus produces false-positive results.

Conclusions

The prevalence of reovirus type-3 might be overestimated in German animal facilities and most likely in other countries as well. The occurrence of other reovirus serotypes, however, raises the question if murine health monitoring programs should be extended to these pathogens.

Electronic supplementary material

The online version of this article (10.1186/s12985-018-1021-8) contains supplementary material, which is available to authorized users.

The regulation of skeletal muscle fiber-type composition by betaine is associated with NFATc1/MyoD

Increasing evidence indicates that muscular dysfunction or alterations in skeletal muscle fiber-type composition not only are involved in muscle metabolism and function but also can limit functional capacity. Therefore, understanding the mechanisms regulating key events during skeletal myogenesis is necessary. Betaine is a naturally occurring component of commonly eaten foods. Here, we showed that 10 mM betaine supplementation in vitro significantly repressed myoblast proliferation and enhanced myoblast differentiation. This effect can be mediated by regulation of miR-29b-3p. Further analysis showed that betaine supplementation in vitro regulated skeletal muscle fiber-type composition through the induction of NFATc1 and the negative regulation of MyoD expression. Furthermore, mice fed with 10 mM betaine in water for 133 days showed no impairment in overall health. Consistently, betaine supplementation increased muscle mass, promoted muscle formation, and modulated the ratio of fiber types in skeletal muscle in vivo. These findings shed light on the diverse biological functions of betaine and indicate that betaine supplementation may lead to new therapies for diseases such as muscular dystrophy or other diseases related to muscle dysfunction. KEY MESSAGES: Betaine supplementation inhibits proliferation and promotes differentiation of C2C12 myoblasts. Betaine supplementation regulates fast to slow muscle fiber-type conversion and is associated with NFATc1/MyoD. Betaine supplementation enhances skeletal myogenesis in vivo. Betaine supplementation does not impair health of mice.

Bovine herpesvirus type 5 replication and induction of apoptosis in vitro and in the trigeminal ganglion of experimentally-infected cattle

Bovine herpesvirus (BoHV) types 1 and 5 are neuroinvasive. Cases of BoHV-1-induced encephalitis are not as frequent as those caused by BoHV-5. In this study, the capability of BoHV-5 to induce apoptosis in cell cultures and in the trigeminal ganglion during acute infection of experimentally-infected cattle was analyzed. Apoptotic changes in cell cultures agree with the ability of the viral strains to replicate in each cell line. Marked differences were observed between the in vitro induction of apoptosis by BoHV-1Cooper and BoHV-5 97/613 strains. Apoptotic neurons were clearly evident in the trigeminal ganglion of BoHV-1-infected calves. For BoHV-5 a fewer number of positive neurons was observed. There is an association between the magnitude of bovine herpesviruses replication and the induction of apoptosis in trigeminal ganglion. These findings suggest that the induction of apoptosis and the innate immune response orchestrate the final outcome of alpha herpesviruses infection of the bovine nervous system.

An Orchestra of Reovirus Receptors: Still Searching for the Conductor

Viruses are constantly engaged in a molecular arms race with the host, where efficient and tactical use of cellular receptors benefits critical steps in infection. Receptor use dictates initiation, establishment, and spread of viral infection to new tissues and hosts. Mammalian orthoreoviruses (reoviruses) are pervasive pathogens that use multiple receptors to overcome protective host barriers to disseminate from sites of initial infection and cause disease in young mammals. In particular, reovirus invades the central nervous system (CNS) with serotype-dependent tropism and disease. A single viral gene, encoding the attachment protein σ1, segregates with distinct patterns of CNS injury. Despite the identification and characterization of several reovirus receptors, host factors that dictate tropism via interaction with σ1 remain undefined. Here, we summarize the state of the reovirus receptor field and discuss open questions toward understanding how the reovirus attachment protein dictates CNS tropism.

Mitochondrial p53 Contributes to Reovirus-Induced Neuronal Apoptosis and Central Nervous System Injury in a Mouse Model of Viral Encephalitis

The tumor suppressor p53 plays a critical part in determining cell fate both as a regulator of the transcription of several proapoptotic genes and through its binding interactions with Bcl-2 family proteins at mitochondria. We now demonstrate that p53 protein levels are increased in infected brains during reovirus encephalitis. This increase occurs in the cytoplasm of reovirus-infected neurons and is associated with the activation of caspase 3. Increased levels of p53 in reovirus-infected brains are not associated with increased expression levels of p53 mRNA, suggesting that p53 regulation occurs at the protein level. Increased levels of p53 are also not associated with the increased expression levels of p53-regulated, proapoptotic genes. In contrast, upregulated p53 accumulates in mitochondria. Previous reports demonstrated that the binding of p53 to Bak at mitochondria causes Bak activation and results in apoptosis. We now show that Bak is activated and that activated Bak is bound to p53 during reovirus encephalitis. In addition, survival is enhanced in reovirus-infected Bak(-/-) mice compared to controls, demonstrating a role for Bak in reovirus pathogenesis. Inhibition of the mitochondrial translocation of p53 with pifithrin μ prevents the formation of p53/Bak complexes following reovirus infection of ex vivo brain slice cultures and results in decreased apoptosis and tissue injury. These results suggest that the mitochondrial localization of p53 regulates reovirus-induced pathogenesis in the central nervous system (CNS) through its interactions with Bak.

IMPORTANCE

There are virtually no specific treatments of proven efficacy for virus-induced neuroinvasive diseases. A better understanding of the pathogenesis of virus-induced CNS injury is crucial for the rational development of novel therapies. Our studies demonstrate that p53 is activated in the brain following reovirus infection and may provide a therapeutic target for virus-induced CNS disease.

The Nogo receptor NgR1 mediates infection by mammalian reovirus

Neurotropic viruses, including mammalian reovirus, must disseminate from an initial site of replication to the central nervous system (CNS), often binding multiple receptors to facilitate systemic spread. Reovirus engages junctional adhesion molecule A (JAM-A) to disseminate hematogenously. However, JAM-A is dispensable for reovirus replication in the CNS. We demonstrate that reovirus binds Nogo receptor NgR1, a leucine-rich repeat protein expressed in the CNS, to infect neurons. Expression of NgR1 confers reovirus binding and infection of nonsusceptible cells. Incubating reovirus virions with soluble NgR1 neutralizes infectivity. Blocking NgR1 on transfected cells or primary cortical neurons abrogates reovirus infection. Concordantly, reovirus infection is ablated in primary cortical neurons derived from NgR1 null mice. Reovirus virions bind to soluble JAM-A and NgR1, while infectious disassembly intermediates (ISVPs) bind only to JAM-A. These results suggest that reovirus uses different capsid components to bind distinct cell-surface molecules, engaging independent receptors to facilitate spread and tropism.

Reovirus-mediated induction of ADAR1 (p150) minimally alters RNA editing patterns in discrete brain regions

Transcripts encoding ADAR1, a double-stranded, RNA-specific adenosine deaminase involved in the adenosine-to-inosine (A-to-I) editing of mammalian RNAs, can be alternatively spliced to produce an interferon-inducible protein isoform (p150) that is up-regulated in both cell culture and in vivo model systems in response to pathogen or interferon stimulation. In contrast to other tissues, p150 is expressed at extremely low levels in the brain and it is unclear what role, if any, this isoform may play in the innate immune response of the central nervous system (CNS) or whether the extent of editing for RNA substrates critical for CNS function is affected by its induction. To investigate the expression of ADAR1 isoforms in response to viral infection and subsequent alterations in A-to-I editing profiles for endogenous ADAR targets, we used a neurotropic strain of reovirus to infect neonatal mice and quantify A-to-I editing in discrete brain regions using a multiplexed, high-throughput sequencing strategy. While intracranial injection of reovirus resulted in a widespread increase in the expression of ADAR1 (p150) in multiple brain regions and peripheral organs, significant changes in site-specific A-to-I conversion were quite limited, suggesting that steady-state levels of p150 expression are not a primary determinant for modulating the extent of editing for numerous ADAR targets in vivo.

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.

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.

Daxx upregulation within the cytoplasm of reovirus-infected cells is mediated by interferon and contributes to apoptosis

Reovirus infection is a well-characterized experimental system for the study of viral pathogenesis and antiviral immunity within the central nervous system (CNS). We have previously shown that c-Jun N-terminal kinase (JNK) and the Fas death receptor each play a role in neuronal apoptosis occurring in reovirus-infected brains. Death-associated protein 6 (Daxx) is a cellular protein that mechanistically links Fas signaling to JNK signaling in several models of apoptosis. In the present study, we demonstrate that Daxx is upregulated in reovirus-infected brain tissue through a type I interferon-mediated mechanism. Daxx upregulation is limited to brain regions that undergo reovirus-induced apoptosis and occurs in the cytoplasm and nucleus of neurons. Cytoplasmic Daxx is present in Fas-expressing cells during reovirus encephalitis, suggesting a role for Daxx in Fas-mediated apoptosis following reovirus infection. Further, in vitro expression of a dominant negative form of Daxx (DN-Daxx), which binds to Fas but which does not transmit downstream signaling, inhibits apoptosis of reovirus-infected cells. In contrast, in vitro depletion of Daxx results in increased expression of caspase 3 and apoptosis, suggesting that Daxx plays an antiapoptotic role in the nucleus. Overall, these data imply a regulatory role for Daxx in reovirus-induced apoptosis, depending on its location in the nucleus or cytoplasm.

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.

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.

Type I interferon signaling limits reoviral tropism within the brain and prevents lethal systemic infection

In vivo and ex vivo models of reoviral encephalitis were utilized to delineate the contribution of type I interferon (IFN) to the host’s defense against local central nervous system (CNS) viral infection and systemic viral spread. Following intracranial (i.c.) inoculation with either serotype 3 (T3) or serotype 1 (T1) reovirus, increased expression of IFN-α, IFN-β, and myxovirus-resistance protein (Mx1; a prototypical IFN stimulated gene) was observed in mouse brain tissue. Type I IFN receptor deficient mice (IFNAR^−/−) had accelerated lethality, compared to wildtype (B6wt) controls, following i.c. T1 or T3 challenge. Although viral titers in the brain and eyes of reovirus infected IFNAR^−/− mice were significantly increased, these mice did not develop neurologic signs or brain injury. In contrast, increased reovirus titers in peripheral tissues (liver, spleen, kidney, heart, and blood) of IFNAR^−/− mice were associated with severe intestinal and liver injury. These results suggest that reovirus-infected IFNAR^−/− mice succumb to peripheral disease rather than encephalitis per se. To investigate the potential role of type I IFN in brain tissue, brain slice cultures (BSCs) were prepared from IFNAR^−/− mice and B6wt controls for ex vivo T3 reovirus infection. Compared to B6wt controls, reoviral replication and virus-induced apoptosis were enhanced in IFNAR^−/− BSCs indicating that a type I IFN response, initiated by resident CNS cells, mediates innate viral immunity within the brain. T3 reovirus tropism was extended in IFNAR^−/− brains to include dentate neurons, ependymal cells, and meningeal cells indicating that reovirus tropism within the CNS is dependent upon type I interferon signaling.

The proapoptotic Bcl-2 protein Bax plays an important role in the pathogenesis of reovirus encephalitis

Encephalitis induced by reovirus serotype 3 (T3) strains results from the apoptotic death of infected neurons. Extrinsic apoptotic signaling is activated in reovirus-infected neurons in vitro and in vivo, but the role of intrinsic apoptosis signaling during encephalitis is largely unknown. Bax plays a key role in intrinsic apoptotic signaling in neurons by allowing the release of mitochondrial cytochrome c. We found Bax activation and cytochrome c release in neurons following infection of neonatal mice with T3 reoviruses. Bax(-/-) mice infected with T3 Abney (T3A) have reduced central nervous system (CNS) tissue injury and decreased apoptosis, despite viral replication that is similar to that in wild-type (WT) Bax(+/+) mice. In contrast, in the heart, T3A-infected Bax(-/-) mice have viral growth, caspase activation, and injury comparable to those in WT mice, indicating that the role of Bax in pathogenesis is organ specific. Nonmyocarditic T3 Dearing (T3D)-infected Bax(-/-) mice had delayed disease and enhanced survival compared to WT mice. T3D-infected Bax(-/-) mice had significantly lower viral titers and levels of activated caspase 3 in the brain despite unaffected transneuronal spread of virus. Cytochrome c and Smac release occurred in some reovirus-infected neurons in the absence of Bax; however, this was clearly reduced compared to levels seen in Bax(+/+) wild-type mice, indicating that Bax is necessary for efficient activation of proapoptotic mitochondrial signaling in infected neurons. Our studies suggest that Bax is important for reovirus growth and pathogenesis in neurons and that the intrinsic pathway of apoptosis, mediated by Bax, is important for full expression of disease, CNS tissue injury, apoptosis, and viral growth in the CNS of reovirus-infected mice.

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.

A brain slice culture model of viral encephalitis reveals an innate CNS cytokine response profile and the therapeutic potential of caspase inhibition

Viral encephalitis is a significant cause of human morbidity and mortality in large part due to suboptimal diagnosis and treatment. Murine reovirus infection serves as a classic experimental model of viral encephalitis. Infection of neonatal mice with T3 reoviruses results in lethal encephalitis associated with neuronal infection, apoptosis, and CNS tissue injury. We have developed an ex vivo brain slice culture (BSC) system that recapitulates the basic pathological features and kinetics of viral replication seen in vivo. We utilize the BSC model to identify an innate, brain-tissue specific inflammatory cytokine response to reoviral infection, which is characterized by the release of IL6, CXCL10, RANTES, and murine IL8 analog (KC). Additionally, we demonstrate the potential utility of this system as a pharmaceutical screening platform by inhibiting reovirus-induced apoptosis and CNS tissue injury with the pan-caspase inhibitor, Q-VD-OPh. Cultured brain slices not only serve to model events occurring during viral encephalitis, but can also be utilized to investigate aspects of pathogenesis and therapy that are not experimentally accessible in vivo.

Attenuated reovirus displays oncolysis with reduced host toxicity

BACKGROUND

Although the naturally occurring reovirus causes only mild symptoms in humans, it shows considerable potential as an oncolytic agent because of its innate ability to target cancer cells. In immunocompromised hosts, however, wild-type reovirus can target healthy tissues, including heart, liver, pancreas and neural structures.

METHODS

We characterized an attenuated form of reovirus (AV) derived from a persistently infected cell line through sequence analysis, as well as western blot and in vitro transcription and translation techniques. To examine its pathogenesis and oncolytic potential, AV reovirus was tested on healthy embryonic stem cells, various non-transformed and transformed cell lines, and in severe combined immunodeficiency (SCID) mice with tumour xenografts.

RESULTS

Sequence analysis of AV reovirus revealed a premature STOP codon in its sigma 1 attachment protein. Western blot and in vitro translation confirmed the presence of a truncated σ1. In comparison to wild-type reovirus, AV reovirus did not kill healthy stem cells or induce black tail formation in SCID mice. However, it did retain its ability to target cancer cells and reduce tumour size.

CONCLUSION

Despite containing a truncated attachment protein, AV reovirus still preferentially targets cancer cells, and compared with wild-type reovirus it shows reduced toxicity when administered to immunodeficient hosts, suggesting the potential use of AV reovirus in combination cancer therapy.

Two overlapping domains of a lyssavirus matrix protein that acts on different cell death pathways

The lyssavirus matrix (M) protein induces apoptosis. The regions of the M protein that are essential for triggering cell death pathways are not yet clearly defined. We therefore compared the M proteins from two viruses that have contrasting characteristics in terms of cellular apoptosis: a genotype 3 lyssavirus, Mokola virus (MOK), and a genotype 1 rabies virus isolated from a dog from Thailand (THA). We identified a 20-amino-acid fragment (corresponding to positions 67 to 86) that retained the cell death activities of the full-length M protein from MOK via both the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and inhibition of cytochrome c oxidase (CcO) activity. We found that the amino acids at positions 77 and 81 have an essential role in triggering these two cell death pathways. Directed mutagenesis demonstrated that the amino acid at position 77 affects CcO activity, whereas the amino acid at position 81 affects TRAIL-dependent apoptosis. Mutations in the full-length M protein that compromised induction of either of these two pathways resulted in delayed apoptosis compared with the time to apoptosis for the nonmutated control.

Gene expression in the brain during reovirus encephalitis

Viral encephalitis remains a significant cause of morbidity and mortality throughout the world. We performed microarray analysis to identify genes and pathways that are differentially regulated during reovirus encephalitis and that may provide novel therapeutic targets for virus-induced diseases of the central nervous system (CNS). An increase in the expression of 130 cellular genes was found in the brains of reovirus-infected mice at early times post infection, compared to mock-infected controls. The up-regulation of these genes was consistent with activation of innate immune responses, particularly interferon signaling. At later times post infection, when significant CNS injury is present and mice exhibit signs of severe neurologic disease, many more (1374) genes were up-regulated, indicating that increased gene expression correlates with disease pathology. Virus-induced gene expression at late times post infection was again consistent with the activation of innate immune responses. However, additional significant pathways included those associated with cytokine signaling and apoptosis, both of which can contribute to CNS injury. This is the first report comparing virus-induced cellular gene and pathway regulation at early and late times following virus infection of the brain. The shift of virus-induced gene expression from innate immune responses at early times post infection to cytokine signaling and apoptosis at later times suggests a potential therapeutic strategy that preserves early protective responses whilst inhibiting later responses that contribute to pathogenesis.

Fas-mediated apoptotic signaling in the mouse brain following reovirus infection

Type 3 (T3) reovirus strains induce apoptotic neuronal cell death and lethal encephalitis in infected mice. T3 strain Dearing (T3D)-induced apoptosis in primary neuronal cultures occurs by a Fas-mediated mechanism and requires the activation of caspase 8. We now show that Fas mRNA is upregulated in the brains of mice infected with encephalitic reovirus T3D and T3 strain Abney (T3A) but not following infection with nonencephalitic reovirus type 1 strain Lang. Fas is upregulated in regions of the brain that are injured during infection with T3 reovirus strains and colocalizes with virus antigen in individual neurons. In contrast, levels of FasL mRNA induced by encephalitic and nonencephalitic reovirus strains do not differ significantly. Caspase 8, the initiator caspase associated with Fas-mediated apoptosis, is activated in the cortex and hippocampal regions of both T3D- and T3A-infected mice. Furthermore, Bid cleavage and the activation of caspase 9 in the brains of T3D-infected mice suggest that the caspase 8-dependent activation of mitochondrial apoptotic signaling contributes to virus-induced apoptosis. We have previously shown that the inhibition of c-Jun N-terminal kinase (JNK) signaling blocks T3D-induced apoptosis and improves the outcome of virus-induced encephalitis. We now show that the reovirus-induced upregulation of Fas requires JNK signaling, thereby providing a link between reovirus-induced death receptor signaling and mitogen-activated protein kinase pathways and a potential mechanism for the therapeutic action of JNK inhibition.

Reovirus activates transforming growth factor beta and bone morphogenetic protein signaling pathways in the central nervous system that contribute to neuronal survival following infection

Viral infections of the central nervous system (CNS) are important causes of worldwide morbidity and mortality, and understanding how viruses perturb host cell signaling pathways will facilitate identification of novel antiviral therapies. We now show that reovirus infection activates transforming growth factor beta (TGF-beta) and bone morphogenetic protein (BMP) signaling in a murine model of encephalitis in vivo. TGF-beta receptor I (TGF-beta RI) expression is increased and its downstream signaling factor, SMAD3, is activated in the brains of reovirus-infected mice. TGF-beta signaling is neuroprotective, as inhibition with a TGF-beta RI inhibitor increases death of infected neurons. Similarly, BMP receptor I expression is increased and its downstream signaling factor, SMAD1, is activated in reovirus-infected neurons in the brains of infected mice in vivo. Activated SMAD1 and SMAD3 were both detected in regions of brain infected by reovirus, but activated SMAD1 was found predominantly in uninfected neurons in close proximity to infected neurons. Treatment of reovirus-infected primary mouse cortical neurons with a BMP agonist reduced apoptosis. These data provide the first evidence for the activation of TGF-beta and BMP signaling pathways following neurotropic viral infection and suggest that these signaling pathways normally function as part of the host's protective innate immune response against CNS viral infection.

Apoptosis in animal models of virus-induced disease

Apoptosis is associated with virus-induced human diseases of the central nervous system, heart and liver, and causes substantial morbidity and mortality. Although virus-induced apoptosis is well characterized in individual cells in cell culture, virus-induced apoptosis in vivo and the role of apoptosis in virus-induced disease is not well established. This review focuses on animal models of virus-induced diseases of the central nervous system, heart and liver that provide insights into the role of apoptosis in pathogenesis, the pathways involved and the potential therapeutic implications.

Experimental reovirus-induced acute flaccid paralysis and spinal motor neuron cell death

Acute flaccid paralysis (AFP) describes the loss of motor function in 1 or more limbs commonly associated with viral infection and destruction of motor neurons in the anterior horns of the spinal cord. Therapy is limited, and the development of effective treatments is hampered by a lack of experimental models. Reovirus infection of neonatal mice provides a model for the study of CNS viral infection pathogenesis. Injection of the Reovirus serot Type 3 strains Abney (T3A) or Dearing (T3D) into the hindlimb of 1-day-old mice resulted in the development of AFP in more than 90% of infected mice. Acute flaccid paralysis began in the ipsilateral hindlimb at 8 to 10 days postinfection and progressed to paraplegia 24 hours later. Paralysis correlated with injury, neuron loss, and spread of viral antigen first to the ipsilateral and then to the contralateral anterior horns. As demonstrated by the activation of caspase 3 and its colocalization with viral antigen in the anterior horn and concomitant cleavage of poly-(adenosine diphosphate-ribose) polymerase, AFP was associated with apoptosis. Calpain activity and inducible nitric oxide synthase expression were both elevated in the spinal cords of paralyzed animals. This study represents the first detailed characterization of a novel and highly efficient experimental model of virus-induced AFP that will facilitate evaluation of therapeutic strategies targeting virus-induced paralysis.

Retinoic acid-inducible gene-I and interferon-beta promoter stimulator-1 augment proapoptotic responses following mammalian reovirus infection via interferon regulatory factor-3

During viral infection, cells initiate antiviral responses to contain replication and inhibit virus spread. One protective mechanism involves activation of transcription factors interferon regulatory factor-3 (IRF-3) and NF-kappaB, resulting in secretion of the antiviral cytokine, interferon-beta. Another is induction of apoptosis, killing the host cell before virus disseminates. Mammalian reovirus induces both interferon-beta and apoptosis, raising the possibility that both pathways are initiated by a common cellular sensor. We show here that reovirus activates IRF-3 with kinetics that parallel the activation of NF-kappaB, a known mediator of reovirus-induced apoptosis. Activation of IRF-3 requires functional retinoic acid inducible gene-I and interferon-beta promoter stimulator-1, but these intracellular sensors are dispensable for activation of NF-kappaB. Interferon-beta promoter stimulator-1 and IRF-3 are required for efficient apoptosis following reovirus infection, suggesting a common mechanism of antiviral cytokine induction and activation of the cell death response.

Novel strategy for treatment of viral central nervous system infection by using a cell-permeating inhibitor of c-Jun N-terminal kinase

Viral encephalitis is a major cause of morbidity and mortality worldwide, yet there is no proven efficacious therapy for most viral infections of the central nervous system (CNS). Many of the viruses that cause encephalitis induce apoptosis and activate c-Jun N-terminal kinase (JNK) following infection. We have previously shown that reovirus infection of epithelial cell lines activates JNK-dependent apoptosis. We now show that reovirus infection resulted in activation of JNK and caspase-3 in the CNS. Treatment of reovirus-infected mice with a cell-permeating peptide that competitively inhibits JNK activity resulted in significantly prolonged survival of intracerebrally infected mice following an otherwise lethal challenge with T3D (100 x 50% lethal dose). Protection correlated with reduced CNS injury, reduced neuronal apoptosis, and reduced c-Jun activation without altering the viral titer or viral antigen distribution. Given the efficacy of the inhibitor in protecting mice from viral encephalitis, JNK inhibition represents a promising and novel treatment strategy for viral encephalitis.

The Aedes albopictus inhibitor of apoptosis 1 gene protects vertebrate cells from bluetongue virus-induced apoptosis

We sequenced and characterized the inhibitor of apoptosis (iap) 1 gene from Aedes albopictus, designated as Aaiap1. The Aaiap1 gene rescued Spodoptera frugiperda (Sf9) cells from apoptosis when cotransfected with the Drosophila pro-apoptotic hid gene. The antiapoptotic function of the Aaiap1 gene was evaluated in the bluetongue virus (BTV)-induced apoptosis system. BTV infection induced apoptosis in vertebrate cells via the intrinsic apoptotic pathway. This was shown by the translocation of cytochrome C and the second mitochondria-derived activator of caspase (Smac, also known as DIABLO) from the mitochondria and the subsequent activation of caspase-9 and -3. Stable expression of the Aaiap1 gene in derivative baby hamster kidney cells delayed BTV-induced apoptosis by 24 h and reduced the BTV progeny yield by 10-fold. This study provides the first evidence that the mosquito AaIAP1 protein possesses antiapoptotic activity.

Attachment and cell entry of mammalian orthoreovirus

Mammalian orthoreoviruses (reoviruses) serve as a tractable model system for studies of viral pathogenesis. Reoviruses infect virtually all mammals, but cause disease only in the very young. Prototype strains of the three reovirus serotypes differ in pathogenesis following infection of newborn mice. Reoviruses are nonenveloped, icosahedral particles that consist of ten segments of double-stranded RNA encapsidated within two protein shells, the inner core and outer capsid. High-resolution structures of individual components of the reovirus outer capsid and a single viral receptor have been solved and provide insight into the functions of these molecules in viral attachment, entry, and pathogenesis. Attachment of reovirus to target cells is mediated by the reovirus sigma1 protein, a filamentous trimer that projects from the outer capsid. Junctional adhesion molecule-A is a serotype-independent receptor for reovirus, and sialic acid is a coreceptor for serotype 3 strains. After binding to receptors on the cell surface, reovirus is internalized via receptor-mediated endocytosis. Internalization is followed by stepwise disassembly of the viral outer capsid in the endocytic compartment. Uncoating events, which require acidic pH and endocytic proteases, lead to removal of major outer-capsid protein sigma3, resulting in exposure of membrane-penetration mediator micro1 and a conformational change in attachment protein sigma1. After penetration of endosomes by uncoated particles, the transcriptionally active viral core is released into the cytoplasm, where replication proceeds. Despite major advances in defining reovirus attachment and entry mechanisms, many questions remain. Ongoing research is aimed at understanding serotype-dependent differences in reovirus tropism, viral cell-entry pathways, the individual and corporate roles of acidic pH and proteases in viral entry, and micro1 function in membrane penetration.

IkappaB kinase subunits alpha and gamma are required for activation of NF-kappaB and induction of apoptosis by mammalian reovirus

Reoviruses induce apoptosis both in cultured cells and in vivo. Apoptosis plays a major role in the pathogenesis of reovirus encephalitis and myocarditis in infected mice. Reovirus-induced apoptosis is dependent on the activation of transcription factor NF-kappaB and downstream cellular genes. To better understand the mechanism of NF-kappaB activation by reovirus, NF-kappaB signaling intermediates under reovirus control were investigated at the level of Rel, IkappaB, and IkappaB kinase (IKK) proteins. We found that reovirus infection leads initially to nuclear translocation of p50 and RelA, followed by delayed mobilization of c-Rel and p52. This biphasic pattern of Rel protein activation is associated with the degradation of the NF-kappaB inhibitor IkappaBalpha but not the structurally related inhibitors IkappaBbeta or IkappaBepsilon. Using IKK subunit-specific small interfering RNAs and cells deficient in individual IKK subunits, we demonstrate that IKKalpha but not IKKbeta is required for reovirus-induced NF-kappaB activation and apoptosis. Despite the preferential usage of IKKalpha, both NF-kappaB activation and apoptosis were attenuated in cells lacking IKKgamma/Nemo, an essential regulatory subunit of IKKbeta. Moreover, deletion of the gene encoding NF-kappaB-inducing kinase, which is known to modulate IKKalpha function, had no inhibitory effect on either response in reovirus-infected cells. Collectively, these findings indicate a novel pathway of NF-kappaB/Rel activation involving IKKalpha and IKKgamma/Nemo, which together mediate the expression of downstream proapoptotic genes in reovirus-infected cells.

Reovirus delays diabetes onset but does not prevent insulitis in nonobese diabetic mice

Mice infected with reovirus develop abnormalities in glucose homeostasis. Reovirus strain type 3 Abney (T3A) was capable of systemic infection of nonobese diabetic (NOD) mice, an experimental model of autoimmune diabetes. Reovirus antigen was detected in pancreatic islets of T3A-infected mice, and primary cultures of pancreatic islets from NOD mice supported T3A growth. Significantly fewer T3A-infected animals compared to uninfected controls developed diabetes. However, despite the alteration in diabetes penetrance, insulitis was evident in T3A-infected mice. These results suggest that viral infection of NOD mice alters autoimmune responses to beta-cell antigens and thereby delays development of diabetes.

JAM-A-independent, antibody-mediated uptake of reovirus into cells leads to apoptosis

Apoptosis plays a major role in the cytopathic effect induced by reovirus following infection of cultured cells and newborn mice. Strain-specific differences in the capacity of reovirus to induce apoptosis segregate with the S1 and M2 gene segments, which encode attachment protein sigma1 and membrane penetration protein mu1, respectively. Virus strains that bind to both junctional adhesion molecule-A (JAM-A) and sialic acid are the most potent inducers of apoptosis. In addition to receptor binding, events in reovirus replication that occur during or after viral disassembly but prior to initiation of viral RNA synthesis also are required for reovirus-induced apoptosis. To determine whether reovirus infection initiated in the absence of JAM-A and sialic acid results in apoptosis, Chinese hamster ovary (CHO) cells engineered to express Fc receptors were infected with reovirus using antibodies directed against viral outer-capsid proteins. Fc-mediated infection of CHO cells induced apoptosis in a sigma1-independent manner. Apoptosis following this uptake mechanism requires acid-dependent proteolytic disassembly, since treatment of cells with the weak base ammonium chloride diminished the apoptotic response. Analysis of T1L x T3D reassortant viruses revealed that the mu1-encoding M2 gene segment is the only viral determinant of the apoptosis-inducing capacity of reovirus when infection is initiated via Fc receptors. Additionally, a temperature-sensitive, membrane penetration-defective M2 mutant, tsA279.64, is an inefficient inducer of apoptosis. These data suggest that signaling pathways activated by binding of sigma1 to JAM-A and sialic acid are dispensable for reovirus-mediated apoptosis and that the mu1 protein plays an essential role in stimulating proapoptotic signaling.

Identification of an NF-kappaB-dependent gene network in cells infected by mammalian reovirus

Reovirus infection activates NF-kappaB, which leads to programmed cell death in cultured cells and in the murine central nervous system. However, little is known about how NF-kappaB elicits this cellular response. To identify host genes activated by NF-kappaB following reovirus infection, we used HeLa cells engineered to express a degradation-resistant mutant of IkappaBalpha (mIkappaBalpha) under the control of an inducible promoter. Induction of mIkappaBalpha inhibited the activation of NF-kappaB and blocked the expression of NF-kappaB-responsive genes. RNA extracted from infected and uninfected cells was used in high-density oligonucleotide microarrays to examine the expression of constitutively activated genes and reovirus-stimulated genes in the presence and absence of an intact NF-kappaB signaling axis. Comparison of the microarray profiles revealed that the expression of 176 genes was significantly altered in the presence of mIkappaBalpha. Of these genes, 64 were constitutive and not regulated by reovirus, and 112 were induced in response to reovirus infection. NF-kappaB-regulated genes could be grouped into four distinct gene clusters that were temporally regulated. Gene ontology analysis identified biological processes that were significantly overrepresented in the reovirus-induced genes under NF-kappaB control. These processes include the antiviral innate immune response, cell proliferation, response to DNA damage, and taxis. Comparison with previously identified NF-kappaB-dependent gene networks induced by other stimuli, including respiratory syncytial virus, Epstein-Barr virus, tumor necrosis factor alpha, and heart disease, revealed a number of common components, including CCL5/RANTES, CXCL1/GRO-alpha, TNFAIP3/A20, and interleukin-6. Together, these results suggest a genetic program for reovirus-induced apoptosis involving NF-kappaB-directed expression of cellular genes that activate death signaling pathways in infected cells.

Reovirus infection of the CNS enhances iNOS expression in areas of virus-induced injury

Nitric oxide (NO) has been implicated as a contributor to the host's innate defense against viral infections including those affecting the CNS. Reovirus infection of the CNS is a classic experimental system for understanding the pathogenesis of neurotropic viral infection. Infection with serotype 3 strains is associated with perturbations in various cellular signaling pathways including NF-kappaB and NO plays a regulatory role in many of these same pathways. We therefore examined whether NO production is dysregulated following reovirus serotype 3 strain Abney (T3A) infection of the mouse CNS. Nitric oxide synthase (NOS) activity was significantly higher in brain homogenates from T3A-infected animals compared to mock infected. Increased NOS activity correlated with inducible NOS (iNOS) expression in brain homogenates of T3A-infected animals. Expression of iNOS was confined to areas of viral infection and injury. T3A infection of primary neuronal and glial cultures was also associated with enhanced expression of iNOS. Immunocytochemical studies of primary glial cultures demonstrated that, in addition to its known neuronotropism, T3A was also capable of infecting immature microglial cells. T3A infection did not alter expression of either neuronal or endothelial NOS isoforms in neuronal or glial cultures or in mouse brain. The NO donor S-Nitroso-N-acetyl penicillamine (SNAP) significantly inhibited T3A growth in neuronal cultures, conversely the NOS inhibitor N-omega-Nitro-L-arginine methyl ester hydrochloride (L-NAME) augmented viral growth. Our findings provide the first evidence of reovirus-induced iNOS expression and the first demonstration that NO inhibits mammalian reovirus replication, suggesting that NO may play an antiviral role during reovirus infection.

Mechanisms of reovirus-induced cell death and tissue injury: role of apoptosis and virus-induced perturbation of host-cell signaling and transcription factor activation

Reoviruses have provided insight into the roles played by specific viral genes and the proteins they encode in virus-induced cell death and tissue injury. Apoptosis is a major mechanism of cell death induced by reoviruses. Reovirus-induced apoptosis involves both death-receptor and mitochondrial cell death pathways. Reovirus infection is associated with selective activation of mitogen activated protein kinase (MAPK) cascades including JNK/SAPK. Infection also perturbs transcription factor signaling resulting in the activation of c-Jun and initial activation followed by strain-specific inhibition of NF-kappaB. Infection results in changes in the expression of genes encoding proteins involved in cell cycle regulation, apoptosis, and DNA damage and repair processes. Apoptosis is a major mechanism of reovirus-induced injury to key target organs including the CNS and heart. Inhibition of apoptosis through the use of caspase or calpain inhibitors, minocycline, or in caspase 3(-/-) mice all reduce virus-associated tissue injury and enhance survival of infected animals. Reoviruses induce apoptotic cell death (oncolysis) in a wide variety of cancer cells and tumors. The capacity of reoviruses to grow efficiently in transformed cells is enhanced by the presence of an activated Ras signaling pathway likely through mechanisms involving inhibition of antiviral PKR signaling and activation of Ras/RalGEF/p38 pathways. The potential of reovirus-induced oncolysis in therapy of human cancers is currently being investigated in phase I/II clinical trials.

Involvement of Fas and FasL in Ectromelia virus-induced apoptosis in mouse brain

In this study we showed that the virulent Moscow strain of Ectromelia virus (ECTV-MOS) infection leads to induction of apoptosis in the BALB/c mouse central nervous system. ECTV-MOS-infected cells and inflammation sites were found in brain parenchyma between 5 and 15 days after footpad infection with ECTV-MOS. Infected cells consisted of microglia and monocytes, astrocytes and oligodendrocytes and these type of cells underwent apoptosis within 5-15 days post infection (d.p.i.). The highest number of apoptotic cells was found at 5 and 10 d.p.i. and represented mainly microglia (61.4% and 38.6% of apoptotic cells, respectively) and astrocytes (21% and 8.9%, respectively). The number of apoptotic oligodendrocytes was 5.4% and 4.5%, respectively. Fluorometric assays demonstrated involvement of caspase-1, -3 and -8 but not caspase-9 in apoptosis in ECTV-MOS-infected mouse brains. Expression of Fas/FasL was significantly increased on ECTV-MOS-infected cells between 5 and 15 d.p.i., whereas Fas was up-regulated also on the surrounding, non-infected cells. Taking together we may conclude that ECTV-MOS infection of microglia and astrocytes leads to local inflammation resulting in Fas/FasL up-regulation and apoptosis, which limits mouse central nervous system infection with ECTV-MOS.

Organ-specific roles for transcription factor NF-kappaB in reovirus-induced apoptosis and disease

Reovirus induces apoptosis in cultured cells and in vivo. In cell culture models, apoptosis is contingent upon a mechanism involving reovirus-induced activation of transcription factor NF-kappaB complexes containing p50 and p65/RelA subunits. To explore the in vivo role of NF-kappaB in this process, we tested the capacity of reovirus to induce apoptosis in mice lacking a functional nfkb1/p50 gene. The genetic defect had no apparent effect on reovirus replication in the intestine or dissemination to secondary sites of infection. In comparison to what was observed in wild-type controls, apoptosis was significantly diminished in the CNS of p50-null mice following reovirus infection. In sharp contrast, the loss of p50 was associated with massive reovirus-induced apoptosis and uncontrolled reovirus replication in the heart. Levels of IFN-beta mRNA were markedly increased in the hearts of wild-type animals but not p50-null animals infected with reovirus. Treatment of p50-null mice with IFN-beta substantially diminished reovirus replication and apoptosis, which suggests that IFN-beta induction by NF-kappaB protects against reovirus-induced myocarditis. These findings reveal an organ-specific role for NF-kappaB in the regulation of reovirus-induced apoptosis, which modulates encephalitis and myocarditis associated with reovirus infection.

Minocycline delays disease onset and mortality in reovirus encephalitis

Minocycline is neuroprotective in many experimental models of neurodegenerative diseases and central nervous system (CNS) injury but has not previously been tested in a model of viral encephalitis. Experimental infection of neonatal mice with neurotropic reoviruses is a classic model for studying the pathogenesis of viral encephalitis. Intracerebral inoculation of serotype 3 reovirus strain Dearing (T3D) in neonatal mice results in lethal encephalitis caused by neuronal apoptosis throughout the CNS. Minocycline significantly delayed death in mice to 11.6 +/- 0.9 days post-infection vs. 8.6 +/- 0.7 days post-infection in controls (P < 0.01). Virus-induced CNS injury, apoptosis, viral titer and antigen expression were significantly decreased in the brains of minocycline-treated mice on 6 and 8 days post-infection compared to controls. Virus-induced injury and viral titer in minocycline-treated infected mice at 11 days post-infection were similar to those seen in untreated T3D-infected mice at 8 days post-infection. Little microglial or astrocytic invasion of brain regions with viral injury was found at any time-point in untreated or minocycline-treated mice, suggesting that in this model system the neuroprotective effect exerted by minocycline is more likely due to its anti-apoptotic properties rather than its capacity to inhibit microglial activation and limit gliosis. These findings, similar to those reported for neurodegenerative diseases, indicate that minocycline does not prevent development of fatal reovirus encephalitis but delays disease onset and progression, suggesting that minocycline treatment may provide a useful adjunctive therapy in viral CNS infections.

Inhibition of NF-kappa B activity and cFLIP expression contribute to viral-induced apoptosis

Virus-induced activation of nuclear factor-kappa B (NF-kappaB) is required for Type 3 (T3) reovirus-induced apoptosis. We now show that NF-kappaB is also activated by the prototypic Type 1 reovirus strain Lang (T1L), which induces significantly less apoptosis than T3 viruses, indicating that NF-kappaB activation alone is not sufficient for apoptosis in reovirus-infected cells. A second phase of virus-induced NF-kappaB regulation, where NF-kappaB activation is inhibited at later times following infection with T3 Abney (T3A), is absent in T1L-infected cells. This suggests that inhibition of NF-kappaB activation at later times post infection also contributes to reovirus-induced apoptosis. Reovirus-induced inhibition of stimulus-induced activation of NF-kappaB is significantly associated with apoptosis following infection of HEK293 cells with reassortant reoviruses and is determined by the T3 S1 gene segment, which is also the primary determinant of reovirus-induced apoptosis. Inhibition of stimulus-induced activation of NF-kappaB also occurs following infection of primary cardiac myocytes with apoptotic (8B) but not non-apoptotic (T1L) reoviruses. Expression levels of the NF-kappaB-regulated cellular FLICE inhibitory protein (cFLIP) reflect NF-kappaB activation in reovirus-infected cells. Further, inhibition of NF-kappaB activity and cFLIP expression promote T1L-induced apoptosis. These results demonstrate that inhibition of stimulus-induced activation of NF-kappaB and the resulting decrease in cFLIP expression promote reovirus-induced apoptosis.