Reovirus-induced neuronal apoptosis is mediated by caspase 3 and is associated with the activation of death receptors

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.

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.

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.

Insight into the divergent role of TRAIL in non-neoplastic neurological diseases

Tumour necrosis factor–related apoptosis‐inducing ligand (TRAIL) is a member of the tumour necrosis factor (TNF) superfamily which mainly induces apoptosis of tumour cells and transformed cell lines with no systemic toxicity, whereas they share high sequence homology with TNF and CD95L. These unique effects of TRAIL have made it an important molecule in oncology research. However, the research on TRAIL‐related antineoplastic agents has lagged behind and has been limited by the extensive drug resistance in cancer cells. Given the several findings showing that TRAIL is involved in immune regulation and other pleiotropic biological effects in non‐malignant cells, TRAIL and its receptors have attracted widespread attention from researchers. In the central nervous system (CNS), TRAIL is highly correlated with malignant tumours such as glioma and other non‐neoplastic disorders such as acute brain injury, CNS infection and neurodegenerative disease. Many clinical and animal studies have revealed the dual roles of TRAIL in which it causes damage by inducing cell apoptosis, and confers protection by enhancing both pro‐ and non‐apoptosis effects in different neurological disorders and at different sites or stages. Its pro‐apoptotic effect produces a pro‐survival effect that cannot be underestimated. This review extensively covers in vitro and in vivo experiments and clinical studies investigating TRAIL. It also provides a summary of the current knowledge on the TRAIL signalling pathway and its involvement in pathogenesis, diagnosis and therapeutics of CNS disorders as a basis for future research.

Stigmasterol alleviates cerebral ischemia/reperfusion injury by attenuating inflammation and improving antioxidant defenses in rats

BACKGROUND/AIMS

The paper aimed to investigate the effects of Stigmasterol on inflammatory factors, antioxidant capacity, and apoptotic signaling pathways in brain tissue of rats with cerebral ischemia/reperfusion (I/R) injury.

METHODS

The neurological deficits of the rats were analyzed and HE staining was performed. The cerebral infarct volume was calculated by means of TTC staining, and neuronal apoptosis was detected by TUNEL staining. At the same time, the contents of glutathione peroxidase, glutathione, superoxide dismutase (SOD), nitric oxide, and malondialdehyde in brain tissue were measured. The expression of the relevant protein was detected by means of Western blotting.

RESULTS

The results showed that the neurological deficit score and infarct area of the I/R rats in the soy sterol treatment group were significantly lower than those in the I/R group. Moreover, the levels of carbon monoxide and malondialdehyde in the soysterol group were significantly lower than those in the I/R group, and the expressions of cyclooxygenase-2 (Cox-2) and NF-κB (p65) in the soysterol group were also significantly lower than those in the I/R group. The expression of Nrf2 (nucleus) and heme oxygenase-1 (HO-1) increased significantly, and the activities of antioxidant enzymes and SOD were increased. In addition, the stigmasterol treatment can inhibit apoptosis, down-regulate Bax and cleaved caspase-3 expression, and up-regulate Bcl-Xl expression.

CONCLUSION

Stigmasterol protects the brain from brain I/R damage by reducing oxidative stress and inflammation.

Gamma Interferon Alters Junctional Integrity via Rho Kinase, Resulting in Blood-Brain Barrier Leakage in Experimental Viral Encephalitis

Blood-brain barrier (BBB) breakdown is a hallmark of many diseases of the central nervous system (CNS). Loss of BBB integrity in CNS diseases such as viral encephalitis results in the loss of nutrient/oxygen delivery, rapid infiltration of immune cells, and brain swelling that can exacerbate neuronal injury. Despite this, the cellular and molecular mechanisms that underlie BBB breakdown in viral encephalitis are incompletely understood. We undertook a comprehensive analysis of the cellular and molecular signaling events that induce BBB breakdown in an experimental model of virus-induced encephalitis in which neonatal mice are infected with reovirus (serotype 3 strain Abney). We show that BBB leakage during reovirus infection correlates with morphological changes in the vasculature, reductions in pericytes (BBB supporting cells), and disorganization of vascular junctions. Pathway analysis on RNA sequencing from brain endothelial cells identified the activation of interferon (IFN) signaling within the brain vasculature following reovirus infection. Our in vitro and in vivo studies show that type II IFN mediated by IFN-γ, a well known antiviral signal, is a major contributor to BBB leakage during reovirus infection. We show that IFN-γ reduces barrier properties in cultured brain endothelial cells through Rho kinase (ROCK)-mediated cytoskeletal contractions, resulting in junctional disorganization and cell-cell separations. In vivo neutralization of IFN-γ during reovirus infection significantly improved BBB integrity, pericyte coverage, attenuated vascular ROCK activity, and junctional disorganization. Our work supports a model in which IFN-γ acts directly on the brain endothelium to induce BBB breakdown through a mechanism involving ROCK-induced junctional disorganization.IMPORTANCE In an experimental viral encephalitis mouse model in which mice are infected with reovirus, we show that IFN-γ induces blood-brain barrier leakage. We show that IFN-γ promotes Rho kinase activity, resulting in actin cytoskeletal contractions in the brain endothelium that lead to vascular junctional disorganization and cell-cell separations. These studies now provide insight into a previously unknown mechanism for how blood-brain barrier breakdown occurs in viral encephalitis and implicates IFN-γ-Rho kinase activity as major contributor to this phenomenon. By identifying this mechanism of blood-brain barrier breakdown, we now provide potential therapeutic targets in treating patients with viral causes of encephalitis with the hope of limiting damage to the central nervous system.

Interferon Beta Contributes to Astrocyte Activation in the Brain following Reovirus Infection

Reovirus encephalitis in mice was used as a model system to investigate astrocyte activation (astrogliosis) following viral infection of the brain. Reovirus infection resulted in astrogliosis, as evidenced by increased expression of glial fibrillary acidic protein (GFAP), and the upregulation of genes that have been previously associated with astrocyte activation. Astrocyte activation occurred in regions of the brain that are targeted by reovirus but extended beyond areas of active infection. Astrogliosis also occurred following reovirus infection of ex vivo brain slice cultures (BSCs), demonstrating that factors intrinsic to the brain are sufficient to activate astrocytes and that this process can occur in the absence of any contribution from the peripheral immune response. In agreement with previous reports, reovirus antigen did not colocalize with GFAP in infected brains, suggesting that reovirus does not infect astrocytes. Reovirus-infected neurons produce interferon beta (IFN-β). IFN-β treatment of primary astrocytes resulted in both the upregulation of GFAP and cytokines that are associated with astrocyte activation. In addition, the ability of media from reovirus-infected BSCs to activate primary astrocytes was blocked by anti-IFN-β antibodies. These results suggest that IFN-β, likely released from reovirus-infected neurons, results in the activation of astrocytes during reovirus encephalitis. In areas where infection and injury were pronounced, an absence of GFAP staining was consistent with activation-induced cell death as a mechanism of inflammation control. In support of this, activated Bak and cleaved caspase 3 were detected in astrocytes within reovirus-infected brains, indicating that activated astrocytes undergo apoptosis.IMPORTANCE Viral encephalitis is a significant cause of worldwide morbidity and mortality, and specific treatments are extremely limited. Virus infection of the brain triggers neuroinflammation; however, the role of neuroinflammation in the pathogenesis of viral encephalitis is unclear. Initial neuroinflammatory responses likely contribute to viral clearance, but prolonged exposure to proinflammatory cytokines released during neuroinflammation may be deleterious and contribute to neuronal death and tissue injury. Activation of astrocytes is a hallmark of neuroinflammation. Here, we show that reovirus infection of the brain results in the activation of astrocytes via an IFN-β-mediated process and that these astrocytes later die by Bak-mediated apoptosis. A better understanding of neuroinflammatory responses during viral encephalitis may facilitate the development of new treatment strategies for these diseases.

Inhibition of NET-1 suppresses proliferation and promotes apoptosis of hepatocellular carcinoma cells by activating the PI3K/AKT signaling pathway

The present study aimed to elucidate the underlying mechanism of neuroepithelial cell transforming 1 (NET-1), a member of the Ras homolog gene family, in hepatocellular carcinoma (HCC). To determine the association between the expression of NET-1 and the proliferation and migration of MHCC97-H cells, the cells were transfected with NET-1 small interfering (si)RNA and si negative control. Following transfection with NET-1 siRNA, the proliferation rate of MHCC97-H cells decreased significantly and the percentage of apoptotic cells increased. The HCC cell line MHCC97-H was used in the present study as it exhibited an increased expression level of NET-1 compared with the MHCC97-L cell line. Expression levels of apoptosis-associated proteins including apoptosis regulator Bax (Bax), cyclinD1, apoptosis regulator Bcl-2 (Bcl-2) and caspase-3 were determined. Expression levels of phosphoinositide 3-kinase (PI3K) and protein kinase B (AKT) and their phosphorylated forms were also measured by western blotting. Following NET-1 knockdown, the expression of Bax and cyclinD1 decreased, the expression of Bcl-2 and caspase-3 increased, and the PI3K/AKT signaling pathway was inhibited. The results of the present study suggest that inhibition of NET-1 can suppress the progression of HCC by targeting the PI3K/AKT signaling pathway. NET-1 expression level in HCC cells increased compared with normal liver cells.

Current understanding of reovirus oncolysis mechanisms

Mammalian orthoreovirus (reovirus) is under development as a cancer virotherapy. Clinical trials demonstrate that reovirus-based therapies are safe and tolerated in patients with a wide variety of cancers. Although reovirus monotherapy has proven largely ineffective, reovirus sensitizes cancer cells to existing chemotherapeutic agents and radiation. Clinical trials are underway to test the efficacy of reovirus in combination with chemotherapeutic and radiation regimens and to evaluate the effectiveness of reovirus in conjunction with immunotherapies. Central to the use of reovirus to treat cancer is its capacity to directly kill cancer cells and alter the cellular environment to augment other therapies. Apoptotic cell death is a prominent mechanism of reovirus cancer cell killing. However, reoviruses can also kill cancer cells through nonapoptotic mechanisms. Here, we describe mechanisms of reovirus cancer cell killing, highlight how reovirus is used in combination with existing cancer treatments, and discuss what is known as to how reovirus modulates cancer immunotherapy.

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.

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.

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.

Virus infections in the nervous system

Virus infections usually begin in peripheral tissues and can invade the mammalian nervous system (NS), spreading into the peripheral (PNS) and more rarely the central (CNS) nervous systems. The CNS is protected from most virus infections by effective immune responses and multilayer barriers. However, some viruses enter the NS with high efficiency via the bloodstream or by directly infecting nerves that innervate peripheral tissues, resulting in debilitating direct and immune-mediated pathology. Most viruses in the NS are opportunistic or accidental pathogens, but a few, most notably the alpha herpesviruses and rabies virus, have evolved to enter the NS efficiently and exploit neuronal cell biology. Remarkably, the alpha herpesviruses can establish quiescent infections in the PNS, with rare but often fatal CNS pathology. Here we review how viruses gain access to and spread in the well-protected CNS, with particular emphasis on alpha herpesviruses, which establish and maintain persistent NS infections.

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.

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.

West Nile virus growth is independent of autophagy activation

West Nile virus (WNV) is an arthropod-borne virus with a worldwide distribution that causes neurologic disease and death. Autophagy is a cellular homeostatic mechanism involved in antiviral responses but can be subverted to support viral growth as well. We show that autophagy is induced by WNV infection in cell culture and in primary neuron cultures. Following WNV infection, lysosomes co-localize with autophagosomes resulting in LC3B-II turnover and autolysosomal acidification. However, activation or inhibition of autophagy has no significant effect on WNV growth but pharmacologic inhibition of PI3 kinases associated with autophagy reduce WNV growth. Basal levels of p62/sequestosome1(SQSTM1) do not significantly change following WNV-induced autophagy activation, but p62 is turned over or degraded by autophagy activation implying that p62 expression is increased following WNV-infection. These data show that WNV-induces autophagy but viral growth is independent of autophagy activation suggesting that WNV-specific interactions with autophagy have diverged from other flaviviruses.

Activation of innate immune responses in the central nervous system during reovirus myelitis

Reovirus infection of the murine spinal cord (SC) was used as a model system to investigate innate immune responses during viral myelitis, including the activation of glia (microglia and astrocytes) and interferon (IFN) signaling and increased expression of inflammatory mediators. Reovirus myelitis was associated with the pronounced activation of SC glia, as evidenced by characteristic changes in cellular morphology and increased expression of astrocyte and microglia-specific proteins. Expression of inflammatory mediators known to be released by activated glia, including interleukin-1β (IL-1β), tumor necrosis factor alpha (TNF-α), chemokine (C-C motif) ligand 5 (CCL 5), chemokine (C-X-C motif) ligand 10 (CXCL10), and gamma interferon (IFN-γ), was also significantly upregulated in the SC of reovirus-infected animals compared to mock-infected controls. Reovirus infection of the mouse SC was also associated with increased expression of genes involved in IFN signaling, including IFN-stimulated genes (ISG). Further, reovirus infection of mice deficient in the expression of the IFN-α/β receptor (IFNAR(-/-)) resulted in accelerated mortality, demonstrating that IFN signaling is protective during reovirus myelitis. Experiments performed in ex vivo SC slice cultures (SCSC) confirmed that resident SC cells contribute to the production of at least some of these inflammatory mediators and ISG during reovirus infection. Microglia, but not astrocytes, were still activated, and glia-associated inflammatory mediators were still produced in reovirus-infected INFAR(-/-) mice, demonstrating that IFN signaling is not absolutely required for these neuroinflammatory responses. Our results suggest that activated glia and inflammatory mediators contribute to a local microenvironment that is deleterious to neuronal survival.

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.

Critical role for death-receptor mediated apoptotic signaling in viral myocarditis

BACKGROUND

Apoptosis of cardiac myocytes plays a key role in the pathogenesis of many cardiac diseases, including viral myocarditis. The apoptotic signaling pathways that are activated during viral myocarditis and the role that these pathways play in disease pathogenesis have not been clearly delineated.

METHODS AND RESULTS

We investigated the role of apoptotic signaling pathways after virus infection of primary cardiac myocytes. The death receptor-associated initiator caspase, caspase 8, and the effector caspase, caspase 3, were significantly activated after infection of primary cardiac myocytes with myocarditic, but not non-myocarditic, reovirus strains. Furthermore, reovirus-induced cardiac myocyte apoptosis was significantly inhibited by soluble death receptors. In contrast, the mitochondrial membrane potential remained unaltered and caspase 9, the initiator caspase associated with mitochondrial apoptotic signaling, was only weakly activated in cardiac myocytes after infection with myocarditic reovirus strains. Inhibition of mitochondrial apoptotic signaling had no effect on reovirus-induced cardiac myocyte apoptosis. In accordance with our in vitro data, caspase 8, but not caspase 9, was significantly activated in the hearts of reovirus-infected mice.

CONCLUSIONS

Death receptor, but not mitochondrial, apoptotic signaling plays a key role in apoptosis after infection of cardiac myocytes with myocarditic reovirus strains.

Viral infection and neural stem/progenitor cell's fate: implications in brain development and neurological disorders

Viral infections in the prenatal (during pregnancy) and perinatal period have been a common cause of brain malformation. Besides the immediate neurological dysfunctions, virus infections may critically affect CNS development culminating in long-term cognitive deficits. Most of these neurotropic viruses are most damaging at a critical stage of the host, when the brain is in a dynamic stage of development. The neuropathology can be attributed to the massive neuronal loss induced by the virus as well as lack of CNS repair owing to a deficit in the neural stem/progenitor cell (NSPC) pool or aberrant formation of new neurons from NSPCs. Being one of the mitotically active populations in the post natal brain, the NSPCs have emerged as the potential targets of neurotropic viruses. The NSPCs are self-renewing and multipotent cells residing in the neurogenic niches of the brain, and, therefore, hampering the developmental fate of these cells may adversely affect the overall neurogenesis pattern. A number of neurotropic viruses utilize NSPCs as their cellular reservoirs and often establish latent and persistent infection in them. Both HIV and Herpes virus infect NSPCs over long periods of time and reactivation of the virus may occur later in life. The virus infected NSPCs either undergoes cell cycle arrest or impaired neuronal or glial differentiation, all of which leads to impaired neurogenesis. The disturbances in neurogenesis and CNS development following neurotropic virus infections have direct implications in the viral pathogenesis and long-term neurobehavioral outcome in infected individuals.

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.

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.

Caspase-3 activation is required for reovirus-induced encephalitis in vivo

Reovirus infection of neonatal mice provides a classic experimental system for understanding the molecular pathogenesis of central nervous system (CNS) viral infection. CNS tissue injury, caused by many human neurotropic viruses, including herpes viruses and West Nile virus, is associated with caspase-dependent apoptotic neuronal cell death. We have previously shown that reovirus-induced CNS tissue injury results from apoptosis and is associated with activation of both death-receptor and mitochondrial apoptotic pathways culminating in the activation of the downstream effector caspase, caspase-3. In order to directly investigate the role of caspase-3 in virus-induced neuronal death and CNS tissue injury during encephalitis, we have compared the pathogenesis of reovirus CNS infection in mice lacking the caspase-3 gene (caspase-3 (-/-)) to syngeneic wild-type mice. Prior studies of antiapoptotic treatments for reovirus-infected mice have indicated that protection from reovirus-induced neuronal injury can occur without altering the viral titer in the brains of infected mice. We now show that reovirus infection of caspase-3 (-/-) mice was associated with dramatic reduction in severity of CNS tissue injury, decreased viral antigen and titer in the brain, and enhanced survival of infected mice. Following intracerebral inoculation, the authors also show that virus spread from the brain to the eyes in reovirus-infected caspase-3 (-/-) mice, indicating that viral spread was intact in these mice. Examination of brains of long-term survivors of reovirus infection among caspase-3 (-/-) mice showed that these mice eventually clear their CNS viral infection, and do not manifest residual or delayed CNS tissue injury.

Arboviral encephalitides: transmission, emergence, and pathogenesis

Arthropod-borne viruses (arboviruses) are of paramount concern as a group of pathogens at the forefront of emerging and re-emerging diseases. Although some arboviral infections are asymptomatic or present with a mild influenza-like illness, many are important human and veterinary pathogens causing serious illness ranging from rash and arthritis to encephalitis and hemorrhagic fever. Here, we discuss arboviruses from diverse families (Flaviviruses, Alphaviruses, and the Bunyaviridae) that are causative agents of encephalitis in humans. An understanding of the natural history of these infections as well as shared mechanisms of neuroinvasion and neurovirulence is critical to control the spread of these viruses and for the development of effective vaccines and treatment modalities.

Bid regulates the pathogenesis of neurotropic reovirus

Reovirus infection leads to apoptosis in both cultured cells and the murine central nervous system (CNS). NF-kappaB-driven transcription of proapoptotic cellular genes is required for the effector phase of the apoptotic response. Although both extrinsic death-receptor signaling pathways and intrinsic pathways involving mitochondrial injury are implicated in reovirus-induced apoptosis, mechanisms by which either of these pathways are activated and their relationship to NF-kappaB signaling following reovirus infection are unknown. The proapoptotic Bcl-2 family member, Bid, is activated by proteolytic cleavage following reovirus infection. To understand how reovirus integrates host signaling circuits to induce apoptosis, we examined proapoptotic signaling following infection of Bid-deficient cells. Although reovirus growth was not affected by the absence of Bid, cells lacking Bid failed to undergo apoptosis. Furthermore, we found that NF-kappaB activation is required for Bid cleavage and subsequent proapoptotic signaling. To examine the functional significance of Bid-dependent apoptosis in reovirus disease, we monitored fatal encephalitis caused by reovirus in the presence and absence of Bid. Survival of Bid-deficient mice was significantly enhanced in comparison to wild-type mice following either peroral or intracranial inoculation of reovirus. Decreased reovirus virulence in Bid-null mice was accompanied by a reduction in viral yield. These findings define a role for NF-kappaB-dependent cleavage of Bid in the cell death program initiated by viral infection and link Bid to viral virulence.

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.

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.

JAK-STAT signaling pathways are activated in the brain following reovirus infection

Reovirus infection provides a classic experimental model system for studying the pathogenesis of viral infections of the central nervous system (CNS), with apoptosis acting as the major mechanism of cell death. The authors have examined the role of signal transducer and activator of transcription (STAT)1, a component of Janus-activated kinase (JAK)-STAT signaling, a pathway implicated in antiviral responses and pathways regulating apoptosis, following reovirus infection. Infection of primary cortical neuron cultures with reovirus serotype 3 strain Abney (T3A) resulted in phosphorylation of STAT1 at sites critical for transcriptional activity. Activated STAT1 was also detected in the brain of neonatal mice following T3A infection, with a nuclear pattern of expression in areas of virus-induced injury. Activation of STAT proteins is typically mediated by JAKs. The authors observed JAK2 phosphorylation (Tyr 1007/1008) in brain lysates from T3A-infected mice. Inhibition of JAK activity with the inhibitor AG-490 blocked reovirus-induced STAT1 activation in neuronal cultures, indicating reovirus-induced STAT activation is JAK dependent. Pretreatment of neuronal cultures with antibody raised against interferon (IFN)-alpha/betaR2 inhibited T3A-induced STAT1 phosphorylation, whereas neither IFN-gamma or IFN-gammaR2 antibody pretreatment had any effect on T3A-induced STAT1 phosphorylation. Mice lacking the STAT1 gene demonstrated increased susceptibility to reovirus infection, with increased mortality and higher viral titers in the brain compared to wild-type animals. The results demonstrate activation of a type I IFN-mediated, JAK-dependent STAT signaling pathway following reovirus infection and suggest that STAT1 is a key component of host defense mechanisms against reovirus infection in the brain.

Fas ligand interactions contribute to CD8+ T-cell-mediated control of West Nile virus infection in the central nervous system

West Nile virus (WNV) is a neurotropic flavivirus that causes encephalitis, most frequently in elderly and immunocompromised humans. Previous studies demonstrated that CD8+ T cells utilize perforin-dependent cytolytic mechanisms to limit WNV infection. Nonetheless, the phenotype of perforin-deficient CD8+ T cells was not as severe as that of an absence of CD8+ T cells, suggesting additional effector control mechanisms. In this study, we evaluated the contribution of Fas-Fas ligand (FasL) interactions to CD8+ T-cell-mediated control of WNV infection. Notably, the cell death receptor Fas was strongly upregulated on neurons in culture and in vivo after WNV infection. gld mice that were functionally deficient in FasL expression showed increased susceptibility to lethal WNV infection. Although antigen-specific priming of CD8+ T cells in peripheral lymphoid tissues was normal in gld mice, increased central nervous system (CNS) viral burdens and delayed clearance were observed. Moreover, the adoptive transfer of WNV-primed wild-type but not gld CD8+ T cells to recipient CD8(-/-) or gld mice efficiently limited infection in the CNS and enhanced survival rates. Overall, our data suggest that CD8+ T cells also utilize FasL effector mechanisms to contain WNV infection in Fas-expressing neurons in the CNS.

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.

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.

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.

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.

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.

Diverse apoptotic pathways in enterovirus 71-infected cells

Mechanisms related to the neuropathogenesis of enterovirus 71 infection remain unclear. This investigation conducts a comprehensive study of the apoptotic pathways in neural and non-neural cells following enterovirus 71 infection. Infections with enterovirus 71 not only induce classical cytopathic effects in SF268 (human glioblastoma), SK-N-MC (human neuroblastoma), RD, and Vero cells, but also induce classic signs of apoptosis in all cells, including DNA fragmentation and phosphatidylserine translocation. Apoptosis has also been caused by the efflux of cytochrome c from mitochondria, and subsequently by cleavage of caspase 9 in all cells. Activation of caspase 8 followed by cleavage of the proapoptotic protein Bid only occurs in non-neural cells. Results of this study demonstrate that a mitochondrial pathway of apoptosis mediated by activation and cleavage of caspase 9 is a main pathway in enterovirus 71-induced apoptosis, especially for enterovirus 71-infected neural cells.

Mechanisms of apoptosis during reovirus infection

Reovirus infection has proven to be an excellent experimental system for studying mechanisms of virus-induced pathogenesis. Reoviruses induce apoptosis in a wide variety of cultured cells in vitro and in target tissues in vivo, including the heart and central nervous system. In vivo, viral infection, tissue injury, and apoptosis colocalize, suggesting that apoptosis is a critical mechanism by which disease is triggered in the host. This review examines the mechanisms of reovirus-induced apoptosis and investigates the possibility that inhibition of apoptosis may provide a novel strategy for limiting virus-induced tissue damage following infection.

Nonstructural protein sigma1s is a determinant of reovirus virulence and influences the kinetics and severity of apoptosis induction in the heart and central nervous system

The mechanisms by which viruses kill susceptible cells in target organs and ultimately produce disease in the infected host remain poorly understood. Dependent upon the site of inoculation and strain of virus, experimental infection of neonatal mice with reoviruses can induce fatal encephalitis or myocarditis. Reovirus-induced apoptosis is a major mechanism of tissue injury, leading to disease development in both the brain and heart. In cultured cells, differences in the capacity of reovirus strains to induce apoptosis are determined by the S1 gene segment, which also plays a major role as a determinant of viral pathogenesis in both the heart and the central nervous system (CNS) in vivo. The S1 gene is bicistronic, encoding both the viral attachment protein sigma-1 and the nonstructural protein sigma-1-small (sigma1s). Although sigma1s is dispensable for viral replication in vitro, we wished to investigate the expression of sigma1s in the infected heart and brain and its potential role in reovirus pathogenesis in vivo. Two-day-old mice were inoculated intramuscularly or intracerebrally with either sigma1s(-) or sigma1s(+) reovirus strains. While viral replication in target organs did not differ between sigma1s(-) and sigma1s(+) viral strains, virus-induced caspase-3 activation and resultant histological tissue injury in both the heart and brain were significantly reduced in sigma1s(-) reovirus-infected animals. These results demonstrate that sigma1s is a determinant of the magnitude and extent of reovirus-induced apoptosis in both the heart and CNS and thereby contributes to reovirus pathogenesis and virulence.

Reovirus as an experimental therapeutic for brain and leptomeningeal metastases from breast cancer

Brain and leptomeningeal metastases are common in breast cancer patients and our current treatments are ineffective. Reovirus type 3 is a replication competent, naturally occurring virus that usurps the activated Ras-signaling pathway (or an element thereof) of tumor cells and lyses them but leaves normal cells relatively unaffected. In this study we evaluated reovirus as an experimental therapeutic in models of central nervous system (CNS) metastasis from breast cancer. We found all breast cancer cell lines tested were susceptible to reovirus, with > 50% of these cells lysed within 72 h of infection. In vivo neurotoxicity studies showed only mild local inflammation at the injection site and mild communicating hydrocephalus with neither diffuse encephalitis nor behavioral abnormalities at the therapeutically effective dose of reovirus (intracranial) (ie 10(7) plaque-forming units) or one dose level higher. In vivo, a single intratumoral administration of reovirus significantly reduced the size of tumors established from two human breast cancer cell lines and significantly prolonged survival. Intrathecal administration of reovirus also remarkably prolonged survival in an immunocompetent racine model of leptomeningeal metastases. These data suggest that the evaluation of reovirus as an experimental therapeutic for CNS metastases from breast cancer is warranted.

JNK regulates the release of proapoptotic mitochondrial factors in reovirus-infected cells

Reovirus-induced apoptosis is associated with activation of the proapoptotic mitogen-activated protein kinase c-Jun N-terminal kinase (JNK) and the JNK-associated transcription factor c-Jun. Here we show that reovirus-induced apoptosis and activation of caspase 3 are inhibited in cells deficient in MEK kinase 1, an upstream activator of JNK in reovirus-infected cells. Inhibition of JNK activity following reovirus infection delays the release of proapoptotic mitochondrial factors and the subsequent onset of apoptosis. In contrast, reovirus-induced apoptosis is not blocked by infection with adenovirus expressing dominant-negative c-Jun, and c-Jun activation does not correlate with apoptosis in reovirus-infected cells. This is the first report demonstrating that JNK is associated with regulation of mitochondrial pathways of apoptosis following viral infection.

Linking JNK signaling to NF-kappaB: a key to survival

In addition to marshalling immune and inflammatory responses, transcription factors of the NF-kappaB family control cell survival. This control is crucial to a wide range of biological processes, including B and T lymphopoiesis, adaptive immunity, oncogenesis and cancer chemoresistance. During an inflammatory response, NF-kappaB activation antagonizes apoptosis induced by tumor necrosis factor (TNF)-alpha, a protective activity that involves suppression of the Jun N-terminal kinase (JNK) cascade. This suppression can involve upregulation of the Gadd45-family member Gadd45beta/Myd118, which associates with the JNK kinase MKK7/JNKK2 and blocks its catalytic activity. Upregulation of XIAP, A20 and blockers of reactive oxygen species (ROS) appear to be important additional means by which NF-kappaB blunts JNK signaling. These recent findings might open up entirely new avenues for therapeutic intervention in chronic inflammatory diseases and certain cancers; indeed, the Gadd45beta-MKK7 interaction might be a key target for such intervention.

Does Toll-like receptor 3 play a biological role in virus infections?

The Toll-like receptor (TLR) family functions to recognize conserved microbial and viral structures with the purpose of activating signal pathways to instigate immune responses against infections by these organisms. For example, in vitro studies reveal that the TLR3 ligand is a double-stranded RNA (dsRNA), a product of viral infections. From this observation, it has been proposed that TLR3 is likely an important first signal for virus infections. We approached this issue by investigating the role of TLR3 in four different infectious viral models (lymphocytic choriomeningitis virus (LCMV), vesicular stomatitis virus (VSV), murine cytomegalovirus (MCMV), and reovirus) and in TLR3 genetically deficient ((-/-)) mice. Our results indicate that TLR3 is not universally required for the generation of effective antiviral responses because the absence of TLR3 does not alter either viral pathogenesis or impair host's generation of adaptive antiviral responses to these viruses.