Autophagosome supports coxsackievirus B3 replication in host cells

Absence of autophagy promotes apoptosis by modulating the ROS-dependent RLR signaling pathway in classical swine fever virus-infected cells

A growing number of studies have demonstrated that both macroautophagy/autophagy and apoptosis are important inner mechanisms of cell to maintain homeostasis and participate in the host response to pathogens. We have previously reported that a functional autophagy pathway is trigged by infection of classical swine fever virus (CSFV) and is required for viral replication and release in host cells. However, the interplay of autophagy and apoptosis in CSFV-infected cells has not been clarified. In the present study, we demonstrated that autophagy induction with rapamycin facilitates cellular proliferation after CSFV infection, and that autophagy inhibition by knockdown of essential autophagic proteins BECN1/Beclin 1 or MAP1LC3/LC3 (microtubule-associated protein 1 light chain 3) promotes apoptosis via fully activating both intrinsic and extrinsic mechanisms in CSFV-infected cells. We also found that RIG-I-like receptor (RLR) signaling was amplified in autophagy-deficient cells during CSFV infection, which was closely linked to the activation of the intrinsic apoptosis pathway. Moreover, we discovered that virus infection of autophagy-impaired cells results in an increase in copy number of mitochondrial DNA and in the production of reactive oxygen species (ROS), which plays a significant role in enhanced RLR signaling and the activated extrinsic apoptosis pathway in cultured cells. Collectively, these data indicate that CSFV-induced autophagy delays apoptosis by downregulating ROS-dependent RLR signaling and thus contributes to virus persistent infection in host cells.

Illuminating the Sites of Enterovirus Replication in Living Cells by Using a Split-GFP-Tagged Viral Protein

Enteroviruses induce the formation of membranous structures (replication organelles [ROs]) with a unique protein and lipid composition specialized for genome replication. Electron microscopy has revealed the morphology of enterovirus ROs, and immunofluorescence studies have been conducted to investigate their origin and formation. Yet, immunofluorescence analysis of fixed cells results in a rather static view of RO formation, and the results may be compromised by immunolabeling artifacts. While live-cell imaging of ROs would be preferred, enteroviruses encoding a membrane-anchored viral protein fused to a large fluorescent reporter have thus far not been described. Here, we tackled this constraint by introducing a small tag from a split-GFP system into an RO-resident enterovirus protein. This new tool bridges a methodological gap by circumventing the need for immunolabeling fixed cells and allows the study of the dynamics and formation of enterovirus ROs in living cells.

Like all other positive-strand RNA viruses, enteroviruses generate new organelles (replication organelles [ROs]) with a unique protein and lipid composition on which they multiply their viral genome. Suitable tools for live-cell imaging of enterovirus ROs are currently unavailable, as recombinant enteroviruses that carry genes that encode RO-anchored viral proteins tagged with fluorescent reporters have not been reported thus far. To overcome this limitation, we used a split green fluorescent protein (split-GFP) system, comprising a large fragment [strands 1 to 10; GFP(S1-10)] and a small fragment [strand 11; GFP(S11)] of only 16 residues. The GFP(S11) (GFP with S11 fragment) fragment was inserted into the 3A protein of the enterovirus coxsackievirus B3 (CVB3), while the large fragment was supplied by transient or stable expression in cells. The introduction of GFP(S11) did not affect the known functions of 3A when expressed in isolation. Using correlative light electron microscopy (CLEM), we showed that GFP fluorescence was detected at ROs, whose morphologies are essentially identical to those previously observed for wild-type CVB3, indicating that GFP(S11)-tagged 3A proteins assemble with GFP(S1-10) to form GFP for illumination of bona fide ROs. It is well established that enterovirus infection leads to Golgi disintegration. Through live-cell imaging of infected cells expressing an mCherry-tagged Golgi marker, we monitored RO development and revealed the dynamics of Golgi disassembly in real time. Having demonstrated the suitability of this virus for imaging ROs, we constructed a CVB3 encoding GFP(S1-10) and GFP(S11)-tagged 3A to bypass the need to express GFP(S1-10) prior to infection. These tools will have multiple applications in future studies on the origin, location, and function of enterovirus ROs.

IMPORTANCE Enteroviruses induce the formation of membranous structures (replication organelles [ROs]) with a unique protein and lipid composition specialized for genome replication. Electron microscopy has revealed the morphology of enterovirus ROs, and immunofluorescence studies have been conducted to investigate their origin and formation. Yet, immunofluorescence analysis of fixed cells results in a rather static view of RO formation, and the results may be compromised by immunolabeling artifacts. While live-cell imaging of ROs would be preferred, enteroviruses encoding a membrane-anchored viral protein fused to a large fluorescent reporter have thus far not been described. Here, we tackled this constraint by introducing a small tag from a split-GFP system into an RO-resident enterovirus protein. This new tool bridges a methodological gap by circumventing the need for immunolabeling fixed cells and allows the study of the dynamics and formation of enterovirus ROs in living cells.

Myocarditis

Viral myocarditis remains a prominent infectious-inflammatory disease for patients throughout the lifespan. The condition presents several challenges including varied modes of clinical presentation, a range of timepoints when patients come to attention, a diversity of approaches to diagnosis, a spectrum of clinical courses, and unsettled perspectives on therapeutics in different patient settings and in the face of different viral pathogens. In this review, we examine current knowledge about viral heart disease and especially provide information on evolving understanding of mechanisms of disease and efforts by investigators to identify and evaluate potential therapeutic avenues for intervention.

Protein 2B of Coxsackievirus B3 Induces Autophagy Relying on Its Transmembrane Hydrophobic Sequences

Coxsackievirus B (CVB) belongs to Enterovirus genus within the Picornaviridae family, and it is one of the most common causative pathogens of viral myocarditis in young adults. The pathogenesis of myocarditis caused by CVB has not been completely elucidated. In CVB infection, autophagy is manipulated to facilitate viral replication. Here we report that protein 2B, one of the non-structural proteins of CVB3, possesses autophagy-inducing capability. The autophagy-inducing motif of protein 2B was identified by the generation of truncated 2B and site-directed mutagenesis. The expression of 2B alone was sufficient to induce the formation of autophagosomes in HeLa cells, while truncated 2B containing the two hydrophobic regions of the protein also induced autophagy. In addition, we demonstrated that a single amino acid substitution (56V→A) in the stem loop in between the two hydrophobic regions of protein 2B abolished the formation of autophagosomes. Moreover, we found that 2B and truncated 2B with autophagy-inducting capability were co-localized with LC3-II. This study indicates that protein 2B relies on its transmembrane hydrophobic regions to induce the formation of autophagosomes, while 56 valine residue in the stem loop of protein 2B might exert critical structural influence on its two hydrophobic regions. These results may provide new insight for understanding the molecular mechanism of autophagy triggered by CVB infection.

Mouse Norovirus infection promotes autophagy induction to facilitate replication but prevents final autophagosome maturation

Autophagy is a cellular process used to eliminate intracellular pathogens. Many viruses however are able to manipulate this cellular process for their own advantage. Here we demonstrate that Mouse Norovirus (MNV) infection induces autophagy but does not appear to utilise the autophagosomal membrane for establishment and formation of the viral replication complex. We have observed that MNV infection results in lipidation and recruitment of LC3 to the autophagosome membrane but prevents subsequent fusion of the autophagosomes with lysosomes, as SQSTM1 (an autophagy receptor) accumulates and Lysosome-Associated Membrane Protein1 is sequestered to the MNV replication complex (RC) rather than to autophagosomes. We have additionally observed that chemical modulation of autophagy differentially affects MNV replication. From this study we can conclude that MNV infection induces autophagy, however suppresses the final maturation step of this response, indicating that autophagy induction contributes to MNV replication independently of RC biogenesis.

BPIFB6 Regulates Secretory Pathway Trafficking and Enterovirus Replication

Bactericidal/permeability-increasing protein (BPI) fold-containing family B, member 3 (BPIFB3) is an endoplasmic reticulum (ER)-localized host factor that negatively regulates coxsackievirus B (CVB) replication through its control of the autophagic pathway. Here, we show that another member of the BPIFB family, BPIFB6, functions as a positive regulator of CVB, and other enterovirus, replication by controlling secretory pathway trafficking and Golgi complex morphology. We show that similar to BPIFB3, BPIFB6 localizes exclusively to the ER, where it associates with other members of the BPIFB family. However, in contrast to our findings that RNA interference (RNAi)-mediated silencing of BPIFB3 greatly enhances CVB replication, we show that silencing of BPIFB6 expression dramatically suppresses enterovirus replication in a pan-viral manner. Mechanistically, we show that loss of BPIFB6 expression induces pronounced alterations in retrograde and anterograde trafficking, which correlate with dramatic fragmentation of the Golgi complex. Taken together, these data implicate BPIFB6 as a key regulator of secretory pathway trafficking and viral replication and suggest that members of the BPIFB family participate in diverse host cell functions to regulate virus infections.

IMPORTANCE

Enterovirus infections are associated with a number of severe pathologies, such as aseptic meningitis, dilated cardiomyopathy, type I diabetes, paralysis, and even death. These viruses, which include coxsackievirus B (CVB), poliovirus (PV), and enterovirus 71 (EV71), co-opt the host cell secretory pathway, which controls the transport of proteins from the endoplasmic reticulum to the Golgi complex, to facilitate their replication. Here we report on the identification of a novel regulator of the secretory pathway, bactericidal/permeability-increasing protein (BPI) fold-containing family B, member 6 (BPIFB6), whose expression is required for enterovirus replication. We show that loss of BPIFB6 expression correlates with pronounced defects in the secretory pathway and greatly reduces the replication of CVB, PV, and EV71. Our results thus identify a novel host cell therapeutic target whose function could be targeted to alter enterovirus replication.

Fat(al) attraction: Picornaviruses Usurp Lipid Transfer at Membrane Contact Sites to Create Replication Organelles

All viruses that carry a positive-sense RNA genome (+RNA), such as picornaviruses, hepatitis C virus, dengue virus, and SARS- and MERS-coronavirus, confiscate intracellular membranes of the host cell to generate new compartments (i.e., replication organelles) for amplification of their genome. Replication organelles (ROs) are membranous structures that not only harbor viral proteins but also contain a specific array of hijacked host factors that create a unique lipid microenvironment optimal for genome replication. While some lipids may be locally synthesized de novo, other lipids are shuttled towards ROs. In picornavirus-infected cells, lipids are exchanged at membrane contact sites between ROs and other organelles. In this paper, we review recent advances in our understanding of how picornaviruses exploit host membrane contact site machinery to generate ROs, a mechanism that is used by some other +RNA viruses as well.

Picornaviruses create replication organelles with a unique protein and lipid composition to amplify their genome.

Picornaviruses hijack membrane contact site machinery to shuttle lipids to their replication organelles.

Picornaviruses from different genera employ a cholesterol/PI4P counterflux mechanism to accumulate cholesterol at replication organelles.

Distinct roles of TNF-related apoptosis-inducing ligand (TRAIL) in viral and bacterial infections: from pathogenesis to pathogen clearance

INTRODUCTION

Apoptotic death of different cells observed during infection is thought to limit overwhelming inflammation in response to microbial challenge. However, the underlying apoptotic death mechanisms have not been well defined. Tumor necrosis factor (TNF) related apoptosis-inducing ligand (TRAIL) is a type II transmembrane protein belonging to the TNF superfamily, which is involved not only in tumor growth suppression but in infection control and also in the regulation of both innate and adaptive immune responses.

FINDINGS

In this review, we have summarized data of recent studies on the influence of the TRAIL/TRAIL receptor (TRAIL-R) system on the development of viral and bacterial infections. TRAIL may have a dual function in the immune system being able to kill infected cells and also to participate in the pathogenesis of multiple infections. Moreover, many pathogens have evolved mechanisms to manipulate TRAIL signaling thus increasing pathogen replication.

CONCLUSION

Present data highlight an essential role for the TRAIL/TRAIL-R system in the regulation and modulation of apoptosis and show that TRAIL has distinct roles in pathogenesis and pathogen elimination. Knowledge of the factors that determine whether TRAIL is helpful or harmful supposes its potential therapeutic implications that are only beginning to be explored.

Differential autophagic effects triggered by five different vertebrate iridoviruses in a common, highly permissive mandarinfish fry (MFF-1) cell model

Autophagy of five vertebrate iridoviruses, including one megalocytivirus (infectious spleen and kidney necrosis virus, ISKNV) and four ranaviruses (Chinese giant salamander iridovirus, CGSIV; Tiger frog virus, TFV; Grouper iridovirus, GIV; and Largemouth bass virus, LMBV) were investigated in a common, highly permissive mandarinfish fry (MFF-1) cell model. The results showed marked autophagosome formation in GIV- and LMBV-infected cells but not in ISKNV-, CGSIV- and TFV-infected MFF-1 cells. Strong evidence for the autophagosomes was provided by transmission electron microscopy, the detection of mandarinfish microtubule-associated protein 1 light chain 3B (mLC3)-based fluorescent dot formation and mLC3-I/mLC3-II conversion was provided by Western blotting. Pharmacological tests indicated that autophagy plays an antiviral role during GIV or LMBV infection. Collectively, our data are the first to show that antiviral autophagic effects can be triggered by GIV and LMBV but not by ISKNV, TFV and CGSIV in a common susceptible cell model. These results suggest that differential host-virus interaction strategies may be utilized against different vertebrate iridoviruses; they also indicate the potential effectiveness of an antiviral treatment that modulates autophagy to control iridoviral infections, such as GIV and LMBV.

Membrane dynamics associated with viral infection

Viral replication and spreading are fundamental events in the viral life cycle, accounting for the assembly and egression of nascent virions, events that are directly associated with viral pathogenesis in target hosts. These processes occur in cellular compartments that are modified by specialized viral proteins, causing a rearrangement of different cell membranes in infected cells and affecting the ER, mitochondria, Golgi apparatus, vesicles and endosomes, as well as processes such as autophagic membrane flux. In fact, the activation or inhibition of membrane trafficking and other related activities are fundamental to ensure the adequate replication and spreading of certain viruses. In this review, data will be presented that support the key role of membrane dynamics in the viral cycle, especially in terms of the assembly, egression and infection processes. By defining how viruses orchestrate these events it will be possible to understand how they successfully complete their route of infection, establishing viral pathogenesis and provoking disease. © 2015 The Authors Reviews in Medical Virology Published by John Wiley & Sons, Ltd.

The Autophagic Machinery in Enterovirus Infection

The Enterovirus genus of the Picornaviridae family comprises many important human pathogens, including polioviruses, rhinovirus, enterovirus A71, and enterovirus D68. They cause a wide variety of diseases, ranging from mild to severe life-threatening diseases. Currently, no effective vaccine is available against enteroviruses except for poliovirus. Enteroviruses subvert the autophagic machinery to benefit their assembly, maturation, and exit from host. Some enteroviruses spread between cells via a process described as autophagosome-mediated exit without lysis (AWOL). The early and late phases of autophagy are regulated through various lipids and their metabolizing enzymes. Some of these lipids and enzymes are specifically regulated by enteroviruses. In the present review, we summarize the current understanding of the regulation of autophagic machinery by enteroviruses, and provide updates on recent developments in this field.

Dengue Virus Inhibition of Autophagic Flux and Dependency of Viral Replication on Proteasomal Degradation of the Autophagy Receptor p62

Autophagic flux involves formation of autophagosomes and their degradation by lysosomes. Autophagy can either promote or restrict viral replication. In the case of Dengue virus (DENV), several studies report that autophagy supports the viral replication cycle, and describe an increase of autophagic vesicles (AVs) following infection. However, it is unknown how autophagic flux is altered to result in increased AVs. To address this question and gain insight into the role of autophagy during DENV infection, we established an unbiased, image-based flow cytometry approach to quantify autophagic flux under normal growth conditions and in response to activation by nutrient deprivation or them TOR inhibitor Torin1.We found that DENV induced an initial activation of autophagic flux, followed by inhibition of general and specific autophagy. Early after infection, basal and activated autophagic flux was enhanced. However, during established replication, basal and Torin1-activated autophagic flux was blocked, while autophagic flux activated by nutrient deprivation was reduced, indicating a block to AV formation and reduced AV degradation capacity. During late infection AV levels increased as a result of inefficient fusion of autophagosomes with lysosomes. In addition, endolysosomal trafficking was suppressed, while lysosomal activities were increased.We further determined that DENV infection progressively reduced levels of the autophagy receptor SQSTM1/p62 via proteasomal degradation. Importantly, stable overexpression of p62 significantly suppressed DENV replication, suggesting a novel role for p62 as a viral restriction factor. Overall, our findings indicate that in the course of DENV infection, autophagy shifts from a supporting to an antiviral role, which is countered by DENV.

IMPORTANCE

Autophagic flux is a dynamic process starting with the formation of autophagosomes and ending with their degradation after fusion with lysosomes. Autophagy impacts the replication cycle of many viruses. However, thus far the dynamics of autophagy in case of Dengue virus (DENV) infections has not been systematically quantified. Therefore, we used high-content, imaging-based flow cytometry to quantify autophagic flux and endolysosomal trafficking in response to DENV infection. We report that DENV induced an initial activation of autophagic flux, followed by inhibition of general and specific autophagy. Further, lysosomal activity was increased, but endolysosomal trafficking was suppressed confirming the block of autophagic flux. Importantly, we provide evidence that p62, an autophagy receptor, restrict DENV replication and was specifically depleted in DENV-infected cells via increased proteasomal degradation. These results suggest that during DENV infection autophagy shifts from a proviral to an antiviral cellular process, which is counteracted by the virus.

Ultrastructural Characterization of Turnip Mosaic Virus-Induced Cellular Rearrangements Reveals Membrane-Bound Viral Particles Accumulating in Vacuoles

Positive-strand RNA [(+) RNA] viruses remodel cellular membranes to facilitate virus replication and assembly. In the case of turnip mosaic virus (TuMV), the viral membrane protein 6K2 plays an essential role in endomembrane alterations. Although 6K2-induced membrane dynamics have been widely studied by confocal microscopy, the ultrastructure of this remodeling has not been extensively examined. In this study, we investigated the formation of TuMV-induced membrane changes by chemical fixation and high-pressure freezing/freeze substitution (HPF/FS) for transmission electron microscopy at different times of infection. We observed the formation of convoluted membranes connected to rough endoplasmic reticulum (rER) early in the infection process, followed by the production of single-membrane vesicle-like (SMVL) structures at the midstage of infection. Both SMVL and double-membrane vesicle-like structures with electron-dense cores, as well as electron-dense bodies, were found late in the infection process. Immunogold labeling results showed that the vesicle-like structures were 6K2 tagged and suggested that only the SMVL structures were viral RNA replication sites. Electron tomography (ET) was used to regenerate a three-dimensional model of these vesicle-like structures, which showed that they were, in fact, tubules. Late in infection, we observed filamentous particle bundles associated with electron-dense bodies, which suggests that these are sites for viral particle assembly. In addition, TuMV particles were observed to accumulate in the central vacuole as membrane-associated linear arrays. Our work thus unravels the sequential appearance of distinct TuMV-induced membrane structures for viral RNA replication, viral particle assembly, and accumulation.

IMPORTANCE

Positive-strand RNA viruses remodel cellular membranes for different stages of the infection process, such as protein translation and processing, viral RNA synthesis, particle assembly, and virus transmission. The ultrastructure of turnip mosaic virus (TuMV)-induced membrane remodeling was investigated over several days of infection. The first change that was observed involved endoplasmic reticulum-connected convoluted membrane accumulation. This was followed by the formation of single-membrane tubules, which were shown to be viral RNA replication sites. Later in the infection process, double-membrane tubular structures were observed and were associated with viral particle bundles. In addition, TuMV particles were observed to accumulate in the central vacuole as membrane-associated linear arrays. This work thus unravels the sequential appearance of distinct TuMV-induced membrane structures for viral RNA replication, viral particle assembly, and accumulation.

Ultrastructural Characterization of Turnip Mosaic Virus-Induced Cellular Rearrangements Reveals Membrane-Bound Viral Particles Accumulating in Vacuoles

Positive-strand RNA [(+) RNA] viruses remodel cellular membranes to facilitate virus replication and assembly. In the case of turnip mosaic virus (TuMV), the viral membrane protein 6K2 plays an essential role in endomembrane alterations. Although 6K2-induced membrane dynamics have been widely studied by confocal microscopy, the ultrastructure of this remodeling has not been extensively examined. In this study, we investigated the formation of TuMV-induced membrane changes by chemical fixation and high-pressure freezing/freeze substitution (HPF/FS) for transmission electron microscopy at different times of infection. We observed the formation of convoluted membranes connected to rough endoplasmic reticulum (rER) early in the infection process, followed by the production of single-membrane vesicle-like (SMVL) structures at the midstage of infection. Both SMVL and double-membrane vesicle-like structures with electron-dense cores, as well as electron-dense bodies, were found late in the infection process. Immunogold labeling results showed that the vesicle-like structures were 6K2 tagged and suggested that only the SMVL structures were viral RNA replication sites. Electron tomography (ET) was used to regenerate a three-dimensional model of these vesicle-like structures, which showed that they were, in fact, tubules. Late in infection, we observed filamentous particle bundles associated with electron-dense bodies, which suggests that these are sites for viral particle assembly. In addition, TuMV particles were observed to accumulate in the central vacuole as membrane-associated linear arrays. Our work thus unravels the sequential appearance of distinct TuMV-induced membrane structures for viral RNA replication, viral particle assembly, and accumulation.

IMPORTANCE

Positive-strand RNA viruses remodel cellular membranes for different stages of the infection process, such as protein translation and processing, viral RNA synthesis, particle assembly, and virus transmission. The ultrastructure of turnip mosaic virus (TuMV)-induced membrane remodeling was investigated over several days of infection. The first change that was observed involved endoplasmic reticulum-connected convoluted membrane accumulation. This was followed by the formation of single-membrane tubules, which were shown to be viral RNA replication sites. Later in the infection process, double-membrane tubular structures were observed and were associated with viral particle bundles. In addition, TuMV particles were observed to accumulate in the central vacuole as membrane-associated linear arrays. This work thus unravels the sequential appearance of distinct TuMV-induced membrane structures for viral RNA replication, viral particle assembly, and accumulation.

Neurotropic Enterovirus Infections in the Central Nervous System

Enteroviruses are a group of positive-sense single stranded viruses that belong to the Picornaviridae family. Most enteroviruses infect humans from the gastrointestinal tract and cause mild symptoms. However, several enteroviruses can invade the central nervous system (CNS) and result in various neurological symptoms that are correlated to mortality associated with enteroviral infections. In recent years, large outbreaks of enteroviruses occurred worldwide. Therefore, these neurotropic enteroviruses have been deemed as re-emerging pathogens. Although these viruses are becoming large threats to public health, our understanding of these viruses, especially for non-polio enteroviruses, is limited. In this article, we review recent advances in the trafficking of these pathogens from the peripheral to the central nervous system, compare their cell tropism, and discuss the effects of viral infections in their host neuronal cells.

Enterovirus 71 induces autophagy by regulating has-miR-30a expression to promote viral replication

Enterovirus 71 (EV71), the etiological agent of hand-foot-and-mouth disease, has increasingly become a public health challenge around the world. Previous studies reported that EV71 infection can induce autophagic machinery to enhance viral replication in vitro and in vivo, but did not address the underlying mechanisms. Increasing evidence suggests that autophagy, in a virus-specific manner, may function to degrade viruses or facilitate viral replication. In this study, we reported that EV71 infection of human epidermoid carcinoma (Hep2) and African green monkey kidney cells (Vero) induced autophagy, which is beneficial for viral replication. Our investigation of the mechanisms revealed that EV71 infection resulted in the reduction of cellular miR-30a, which led to the inhibition of Beclin-1, a key autophagy-promoting gene that plays important roles at the early phase of autophagosome formation. We provided further evidence that by modulating cellular miR-30a level through either overexpression or inhibition, one can inhibit or promote EV71 replication, respectively, through regulating autophagic activity.

Aquatic viruses induce host cell death pathways and its application

Virus infections of mammalian and animal cells consist of a series of events. As intracellular parasites, viruses rely on the use of host cellular machinery. Through the use of cell culture and molecular approaches over the past decade, our knowledge of the biology of aquatic viruses has grown exponentially. The increase in aquaculture operations worldwide has provided new approaches for the transmission of aquatic viruses that include RNA and DNA viruses. Therefore, the struggle between the virus and the host for control of the cell's death machinery is crucial for survival. Viruses are obligatory intracellular parasites and, as such, must modulate apoptotic pathways to control the lifespan of their host to complete their replication cycle. This paper updates the discussion on the detailed mechanisms of action that various aquatic viruses use to induce cell death pathways in the host, such as Bad-mediated, mitochondria-mediated, ROS-mediated and Fas-mediated cell death circuits. Understanding how viruses exploit the apoptotic pathways of their hosts may provide great opportunities for the development of future potential therapeutic strategies and pathogenic insights into different aquatic viral diseases.

Inhibition of autophagy in EBV-positive Burkitt's lymphoma cells enhances EBV lytic genes expression and replication

Autophagy, an important degradation system involved in maintaining cellular homeostasis, serves also to eliminate pathogens and process their fragments for presentation to the immune system. Several viruses have been shown to interact with the host autophagic machinery to suppress or make use of this cellular catabolic pathway to enhance their survival and replication. Epstein Barr virus (EBV) is a γ-herpes virus associated with a number of malignancies of epithelial and lymphoid origin in which establishes a predominantly latent infection. Latent EBV can periodically reactivate to produce infectious particles that allow the virus to spread and can lead to the death of the infected cell. In this study, we analyzed the relationship between autophagy and EBV reactivation in Burkitt's lymphoma cells. By monitoring autophagy markers and EBV lytic genes expression, we demonstrate that autophagy is enhanced in the early phases of EBV lytic activation but decreases thereafter concomitantly with increased levels of EBV lytic proteins. In a cell line defective for late antigens expression, we found an inverse correlation between EBV early antigens expression and autophagosomes formation, suggesting that early after activation, the virus is able to suppress autophagy. We report here for the first time that inhibition of autophagy by Bafilomycin A1 or shRNA knockdown of Beclin1 gene, highly incremented EBV lytic genes expression as well as intracellular viral DNA and viral progeny yield. Taken together, these findings indicate that EBV activation induces the autophagic response, which is soon inhibited by the expression of EBV early lytic products. Moreover, our findings open the possibility that pharmacological inhibitors of autophagy may be used to enhance oncolytic viral therapy of EBV-related lymphomas.

Recent advances from studies on the role of structural proteins in enterovirus infection

Enteroviruses are a large group of small nonenveloped viruses that cause common and debilitating illnesses affecting humans and animals worldwide. The capsid composed by viral structural proteins packs the RNA genome. It is becoming apparent that structural proteins of enteroviruses play versatile roles in the virus–host interaction in the viral life cycle, more than just a shell. Furthermore, structural proteins to some extent may be associated with viral virulence and pathogenesis. Better understanding the roles of structural proteins in enterovirus infection may lead to the development of potential antiviral strategies. Here, we discuss recent advances from studies on the role of structural proteins in enterovirus infection and antiviral therapeutics targeted structural proteins.

Coxsackievirus B3 induces autophagy in HeLa cells via the AMPK/MEK/ERK and Ras/Raf/MEK/ERK signaling pathways

In a previous study, the number of autophagosomes increased after coxsackievirus B3 (CVB3) infection. However, the exact mechanism by which CVB3 regulates the number of autophagosomes is unclear. Earlier studies have found that infection with CVB3 activates extracellular signal-regulated kinase (ERK). ERK is essential for CVB3 replication and can increase the number of autophagosomes. In the current study, extracellular signal-regulated kinase 1/2 was activated in HeLa cells after CVB3 infection. The ERK kinase inhibitor, U0126, was then used to inhibit the activity of ERK. Treatment with U0126 led to a significant reduction in the number of autophagosomes indicating that the CVB3-induced autophagosome accumulation may have occurred via the ERK pathway. The relationship between CVB3 infection and ERK pathway activation was also investigated. The results showed that the RasGAP protein could be further cleaved, leading to the activation of the Ras/Raf/MEK (mitogen/extracellular signal-regulated kinase)/ERK pathway and that CVB3 infection could result in an increase in the concentration of calcium in the cytoplasm, resulting in mitochondrial damage, a decrease in the concentration of ATP and activation of the AMPK (AMP-activated protein kinase)/MEK/ERK pathway. In summary, CVB3 might directly or indirectly induce autophagy via AMPK/MEK/ERK and Ras/Raf/MEK/ERK signaling pathways in the host cells, representing a pivotal mechanism for CVB3 pathogenesis.

Inhibition of Histone Deacetylase Activity Aggravates Coxsackievirus B3-Induced Myocarditis by Promoting Viral Replication and Myocardial Apoptosis

Viral myocarditis, which is most prevalently caused by coxsackievirus B3 (CVB3), is a serious clinical condition characterized by excessive myocardial inflammation. Recent studies suggest that regulation of protein acetylation levels by inhibiting histone deacetylase (HDAC) activity modulates inflammatory response and shows promise as a therapy for several inflammatory diseases. However, the role of HDAC activity in viral myocarditis is still not fully understood. Here, we aim to investigate the role of HDAC activity in viral myocarditis and its underlying mechanism. CVB3-infected BALB/c mice were treated with the HDAC inhibitor (HDACI) suberoylanilide hydroxamic acid (SAHA) or trichostatin A (TSA). We found inhibition of HDAC activity aggravated rather than ameliorated the severity of CVB3-induced myocarditis, which was contrary to our expectations. The aggravated myocarditis by HDACI treatment seemed not to be caused by an elevated inflammatory response but by the increased CVB3 replication. Further, it was revealed that the increased CVB3 replication was closely associated with the HDACI-enhanced autophagosome formation. Inhibition of autophagosome formation by wortmannin or ATG5 short hairpin RNA dramatically suppressed the HDACI-increased CVB3 replication. The increased viral replication subsequently elevated CVB3-induced myocardial apoptosis. Conversely, inhibition of CVB3 replication and ensuing myocardial apoptosis by the antiviral drug ribavirin significantly reversed the HDACI-aggravated viral myocarditis. In conclusion, we elucidate that the inhibition of HDAC activity increases CVB3 replication and ensuing myocardial apoptosis, resulting in aggravated viral myocarditis. Possible adverse consequences of administering HDACI should be considered in patients infected (or coinfected) with CVB3.

IMPORTANCE Viral myocarditis, which is most prevalently caused by CVB3, is characterized by excessive myocardial inflammation. Inhibition of HDAC activity was originally identified as a powerful anti-cancer therapeutic strategy and was recently found to be implicated in the regulation of inflammatory response. HDACI has been demonstrated to be efficacious in animal models of several inflammatory diseases. Thus, we hypothesize that inhibition of HDAC activity also protects against CVB3-induced viral myocarditis. Surprisingly, we found inhibition of HDAC activity enhanced myocardial autophagosome formation, which led to the elevated CVB3 viral replication and ensuing increased myocardial apoptosis. Viral myocarditis was eventually aggravated rather than ameliorated by HDAC inhibition. In conclusion, we elucidate the role of HDAC activity in viral myocarditis. Moreover, given the importance of HDACI in preclinical and clinical treatments, the possible unfavorable effect of HDACI should be carefully evaluated in patients infected with viruses, including CVB3.

RIP3 Regulates Autophagy and Promotes Coxsackievirus B3 Infection of Intestinal Epithelial Cells

Receptor interacting protein kinase-3 (RIP3) is an essential kinase for necroptotic cell death signaling and has been implicated in antiviral cell death signaling upon DNA virus infection. Here, we performed high-throughput RNAi screening and identified RIP3 as a positive regulator of coxsackievirus B3 (CVB) replication in intestinal epithelial cells (IECs). RIP3 regulates autophagy, a process utilized by CVB for viral replication factory assembly, and depletion of RIP3 inhibits autophagic flux and leads to the accumulation of autophagosomes and amphisomes. Additionally, later in infection, RIP3 is cleaved by the CVB-encoded cysteine protease 3C(pro), which serves to abrogate RIP3-mediated necrotic signaling and induce a nonnecrotic form of cell death. Taken together, our results show that temporal targeting of RIP3 allows CVB to benefit from its roles in regulating autophagy while inhibiting the induction of necroptotic cell death.

Replication and Inhibitors of Enteroviruses and Parechoviruses

The Enterovirus (EV) and Parechovirus genera of the picornavirus family include many important human pathogens, including poliovirus, rhinovirus, EV-A71, EV-D68, and human parechoviruses (HPeV). They cause a wide variety of diseases, ranging from a simple common cold to life-threatening diseases such as encephalitis and myocarditis. At the moment, no antiviral therapy is available against these viruses and it is not feasible to develop vaccines against all EVs and HPeVs due to the great number of serotypes. Therefore, a lot of effort is being invested in the development of antiviral drugs. Both viral proteins and host proteins essential for virus replication can be used as targets for virus inhibitors. As such, a good understanding of the complex process of virus replication is pivotal in the design of antiviral strategies goes hand in hand with a good understanding of the complex process of virus replication. In this review, we will give an overview of the current state of knowledge of EV and HPeV replication and how this can be inhibited by small-molecule inhibitors.

Recent progress in understanding coxsackievirus replication, dissemination, and pathogenesis

Coxsackieviruses (CVs) are relatively common viruses associated with a number of serious human diseases, including myocarditis and meningo-encephalitis. These viruses are considered cytolytic yet can persist for extended periods of time within certain host tissues requiring evasion from the host immune response and a greatly reduced rate of replication. A member of Picornaviridae family, CVs have been historically considered non-enveloped viruses - although recent evidence suggest that CV and other picornaviruses hijack host membranes and acquire an envelope. Acquisition of an envelope might provide distinct benefits to CV virions, such as resistance to neutralizing antibodies and efficient nonlytic viral spread. CV exhibits a unique tropism for progenitor cells in the host which may help to explain the susceptibility of the young host to infection and the establishment of chronic disease in adults. CVs have also been shown to exploit autophagy to maximize viral replication and assist in unconventional release from target cells. In this article, we review recent progress in clarifying virus replication and dissemination within the host cell, identifying determinants of tropism, and defining strategies utilized by the virus to evade the host immune response. Also, we will highlight unanswered questions and provide future perspectives regarding the potential mechanisms of CV pathogenesis.

The role of autophagy in intracellular pathogen nutrient acquisition

Following entry into host cells intracellular pathogens must simultaneously evade innate host defense mechanisms and acquire energy and anabolic substrates from the nutrient-limited intracellular environment. Most of the potential intracellular nutrient sources are stored within complex macromolecules that are not immediately accessible by intracellular pathogens. To obtain nutrients for proliferation, intracellular pathogens must compete with the host cell for newly-imported simple nutrients or degrade host nutrient storage structures into their constituent components (fatty acids, carbohydrates, and amino acids). It is becoming increasingly evident that intracellular pathogens have evolved a wide variety of strategies to accomplish this task. One recurrent microbial strategy is to exploit host degradative processes that break down host macromolecules into simple nutrients that the microbe can use. Herein we focus on how a subset of bacterial, viral, and eukaryotic pathogens leverage the host process of autophagy to acquire nutrients that support their growth within infected cells.

Activation of AMPK restricts coxsackievirus B3 replication by inhibiting lipid accumulation

Coxsackievirus B3 (CVB3) is the major pathogen of human viral myocarditis. CVB3 has been found to manipulate and modify the cellular lipid metabolism for viral replication. The cellular AMP-activated protein kinase (AMPK) is a key regulator of multiple metabolic pathways, including lipid metabolism. Here we explore the potential roles AMPK plays in CVB3 infection. We found that AMPK is activated by the viral replication during CVB3 infection in Hela cells and primary myocardial cells. RNA interference mediated inhibition of AMPK could increase the CVB3 replication in cells, indicating that AMPK contributed to restricting the viral replication. Next, we showed that CVB3 replication could be inhibited by several different pharmacological AMPK activators including metformin, A769662 and AICAR. And the constitutively active AMPK mutant (CA-AMPK) could also inhibit the CVB3 replication. Furthermore, we found that CVB3 infection increased the cellular lipid levels and showed that the AMPK agonist AICAR both restricted CVB3 replication and reduced lipid accumulation through inhibiting the lipid synthesis associated gene expression. We further found that CVB3 infection would also induce AMPK activated in vivo. The AMPK agonist metformin, which has been widely used in diabetes therapy, could decrease the viral replication and further protect the mice from myocardial histological and functional changes in CVB3 induced myocarditis, and improve the survival rate of infected mice. Lastly, it was demonstrated that the AICAR-mediated restriction of viral replication could be rescued partially by exogenous palmitate, the first product of fatty acid biosynthesis, demonstrating that AMPK activation restricted CVB3 infection through its inhibition of lipid synthesis. Taken together, these data in the present study suggest a model in which AMPK is activated by CVB3 infection and restricts viral replication by inhibiting the cellular lipid accumulation, and inform a potential novel therapeutic strategy for CVB3-associated diseases.

Epigallocatechin-3-gallate opposes HBV-induced incomplete autophagy by enhancing lysosomal acidification, which is unfavorable for HBV replication

Epigallocatechin-3-gallate (EGCG), a major polyphenol in green tea, exhibits diverse beneficial properties, including antiviral activity. Autophagy is a cellular process that is involved in the degradation of long-lived proteins and damaged organelles. Recent evidence indicates that modulation of autophagy is a potential therapeutic strategy for various viral diseases. In the present study, we investigated the effect of EGCG on hepatitis B virus (HBV) replication and the possible involvement of autophagy in this process. Our results showed that HBV induced autophagosome formation, which was required for replication of itself. However, although EGCG efficiently inhibited HBV replication, it enhanced, but not inhibited, autophagosome formation in hepatoma cells. Further study showed that HBV induced an incomplete autophagy, while EGCG, similar to starvation, was able to induce a complete autophagic process, which appeared to be unfavorable for HBV replication. Furthermore, it was found that HBV induced an incomplete autophagy by impairing lysosomal acidification, while it lost this ability in the presence of EGCG. Taken together, these data demonstrated that EGCG treatment opposed HBV-induced incomplete autophagy via enhancing lysosomal acidification, which was unfavorable for HBV replication.

Involvement of Endoplasmic Reticulum Stress-Mediated C/EBP Homologous Protein Activation in Coxsackievirus B3-Induced Acute Viral Myocarditis

BACKGROUND

This study tested the hypothesis whether endoplasmic reticulum (ER) stress/C/EBP homologous protein (CHOP) signaling is linked with coxsackievirus B3 (CVB3)-induced acute viral myocarditis (AVMC) in vivo.

METHODS AND RESULTS

AVMC was induced by intraperitoneal injection of 1000 tissue culture infectious dose (TCID50) of CVB3 virus in mice. In AVMC mouse hearts (n=11), ER stress and CHOP were significantly activated, and were linked to the induction of proapoptotic signaling including reduction of Bcl-2, activation of Bax and caspase 3, compared with the controls (n=10), whereas these could be markedly blocked by ER stress inhibitor tauroursodeoxycholic acid administration (n=11). Moreover, chemical inhibition of ER stress significantly attenuated cardiomyocytes apoptosis, and prevented cardiac troponin I elevation, ameliorated cardiac dysfunction assessed by both hemodynamic and echocardiographic analysis, reduced viral replication, and increased survival rate after CVB3 inoculation. We further discovered that genetic ablation of CHOP (n=10) suppressed cardiac Bcl-2/Bax ratio reduction and caspase 3 activation, and prevented cardiomyotes apoptosis in vivo, compared with wild-type receiving CVB3 inoculation (n=10). Strikingly, CHOP deficiency exhibited dramatic protective effects on cardiac damage, cardiac dysfunction, viral replication, and promoted survival in CVB3-caused AVMC.

CONCLUSIONS

Our data imply the involvement of ER stress/CHOP signaling in CVB3-induced AVMC via proapoptotic pathways, and provide a novel strategy for AVMC treatment.

Cytoplasmic translocation, aggregation, and cleavage of TDP-43 by enteroviral proteases modulate viral pathogenesis

We have previously demonstrated that infection by coxsackievirus B3 (CVB3), a positive-stranded RNA enterovirus, results in the accumulation of insoluble ubiquitin-protein aggregates, which resembles the common feature of neurodegenerative diseases. The importance of protein aggregation in viral pathogenesis has been recognized; however, the underlying regulatory mechanisms remain ill-defined. Transactive response DNA-binding protein-43 (TDP-43) is an RNA-binding protein that has an essential role in regulating RNA metabolism at multiple levels. Cleavage and cytoplasmic aggregation of TDP-43 serves as a major molecular marker for amyotrophic lateral sclerosis and frontotemporal lobar degeneration and contributes significantly to disease progression. In this study, we reported that TDP-43 is translocated from the nucleus to the cytoplasm during CVB3 infection through the activity of viral protease 2A, followed by the cleavage mediated by viral protease 3C. Cytoplasmic translocation of TDP-43 is accompanied by reduced solubility and increased formation of protein aggregates. The cleavage takes place at amino-acid 327 between glutamine and alanine, resulting in the generation of an N- and C-terminal cleavage fragment of ~35 and ~8 kDa, respectively. The C-terminal product of TDP-43 is unstable and quickly degraded through the proteasome degradation pathway, whereas the N-terminal truncation of TDP-43 acts as a dominant-negative mutant that inhibits the function of native TDP-43 in alternative RNA splicing. Lastly, we demonstrated that knockdown of TDP-43 results in an increase in viral titers, suggesting a protective role for TDP-43 in CVB3 infection. Taken together, our findings suggest a novel model by which cytoplasmic redistribution and cleavage of TDP-43 as a consequence of CVB3 infection disrupts the solubility and transcriptional activity of TDP-43. Our results also reveal a mechanism evolved by enteroviruses to support efficient viral infection.

Influence of cellular trafficking pathway on bluetongue virus infection in ovine cells

Bluetongue virus (BTV), a non-enveloped arbovirus, causes hemorrhagic disease in ruminants. However, the influence of natural host cell proteins on BTV replication process is not defined. In addition to cell lysis, BTV also exits non-ovine cultured cells by non-lytic pathways mediated by nonstructural protein NS3 that interacts with virus capsid and cellular proteins belonging to calpactin and ESCRT family. The PPXY late domain motif known to recruit NEDD4 family of HECT ubiquitin E3 ligases is also highly conserved in NS3. In this study using a mixture of molecular, biochemical and microscopic techniques we have analyzed the importance of ovine cellular proteins and vesicles in BTV infection. Electron microscopic analysis of BTV infected ovine cells demonstrated close association of mature particles with intracellular vesicles. Inhibition of Multi Vesicular Body (MVB) resident lipid phosphatidylinositol-3-phosphate resulted in decreased total virus titre suggesting that the vesicles might be MVBs. Proteasome mediated inhibition of ubiquitin or modification of virus lacking the PPXY in NS3 reduced virus growth. Thus, our study demonstrated that cellular components comprising of MVB and exocytic pathways proteins are involved in BTV replication in ovine cells.

Viruses and the autophagy pathway

Studies of the cellular autophagy pathway have exploded over the past twenty years. Now appreciated as a constitutive degradative mechanism that promotes cellular homeostasis, autophagy is also required for a variety of developmental processes, cellular stress responses, and immune pathways. Autophagy certainly acts as both an anti-viral and pro-viral pathway, and the roles of autophagy depend on the virus, the cell type, and the cellular environment. The goal of this review is to summarize, in brief, what we know so far about the relationship between autophagy and viruses, particularly for those who are not familiar with the field. With a massive amount of relevant published data, it is simply not possible to be comprehensive, or to provide a complete "parade of viruses", and apologies are offered to researchers whose work is not described herein. Rather, this review is organized around general themes regarding the relationship between autophagy and animal viruses.

Coxsackievirus B3 replication and pathogenesis

ABSTRACT 

Viruses such as coxsackievirus B3 (CVB3) are entirely host cell-dependent parasites. Indeed, they must cleverly exploit various compartments of host cells to complete their life cycle, and consequently launch disease. Evolution has equipped this pico-rna-virus, CVB3, to use different strategies, including CVB3-induced direct damage to host cells followed by a host inflammatory response to CVB3 infection, and cell death to super-additively promote target organ tissue injury, and dysfunction. In this update, the patho-stratagems of CVB3 are explored from molecular, and systems-level approaches. In summarizing recent developments in this field, we focus particularly on mechanisms by which CVB3 can harness different host cell processes including kinases, host cell-killing and cell-eating machineries, matrix metalloproteinases and miRNAs to promote disease.

HCV induces the expression of Rubicon and UVRAG to temporally regulate the maturation of autophagosomes and viral replication

Hepatitis C virus (HCV) induces autophagy to enhance its replication. However, how HCV regulates the autophagic pathway remains largely unclear. In this report, we demonstrated that HCV infection could induce the expression of Rubicon and UVRAG, which inhibited and stimulated the maturation of autophagosomes, respectively. The induction of Rubicon by HCV was prompt whereas the induction of UVRAG was delayed, resulting in the accumulation of autophagosomes in the early time points of viral infection. The role of Rubicon in inhibiting the maturation of autophagosomes in HCV-infected cells was confirmed by siRNA knockdown and the over-expression of Rubicon, which enhanced and suppressed the maturation of autophagosomes, respectively. Rubicon played a positive role in HCV replication, as the suppression of its expression reduced HCV replication and its over-expression enhanced HCV replication. In contrast, the over-expression of UVRAG facilitated the maturation of autophagosomes and suppressed HCV replication. The HCV subgenomic RNA replicon, which expressed only the nonstructural proteins, could also induce the expression of Rubicon and the accumulation of autophagosomes. Further analysis indicated that the HCV NS4B protein was sufficient to induce Rubicon and autophagosomes. Our results thus indicated that HCV, by differentially inducing the expression of Rubicon and UVRAG, temporally regulated the autophagic flux to enhance its replication.

Itraconazole inhibits enterovirus replication by targeting the oxysterol-binding protein

Itraconazole (ITZ) is a well-known antifungal agent that also has anticancer activity. In this study, we identify ITZ as a broad-spectrum inhibitor of enteroviruses (e.g., poliovirus, coxsackievirus, enterovirus-71, rhinovirus). We demonstrate that ITZ inhibits viral RNA replication by targeting oxysterol-binding protein (OSBP) and OSBP-related protein 4 (ORP4). Consistently, OSW-1, a specific OSBP/ORP4 antagonist, also inhibits enterovirus replication. Knockdown of OSBP inhibits virus replication, whereas overexpression of OSBP or ORP4 counteracts the antiviral effects of ITZ and OSW-1. ITZ binds OSBP and inhibits its function, i.e., shuttling of cholesterol and phosphatidylinositol-4-phosphate between membranes, thereby likely perturbing the virus-induced membrane alterations essential for viral replication organelle formation. ITZ also inhibits hepatitis C virus replication, which also relies on OSBP. Together, these data implicate OSBP/ORP4 as molecular targets of ITZ and point to an essential role of OSBP/ORP4-mediated lipid exchange in virus replication that can be targeted by antiviral drugs.

BPIFB3 regulates autophagy and coxsackievirus B replication through a noncanonical pathway independent of the core initiation machinery

Enteroviruses require autophagy to facilitate the formation of autophagosome (AP)-like double-membrane vesicles that provide the scaffolding for RNA replication. Here, we identify bactericidal/permeability-increasing protein (BPI) fold-containing family B, member 3 (BPIFB3) as a gene whose silencing greatly enhances coxsackievirus B (CVB) replication and induces dramatic alterations in the morphology of CVB-induced replication organelles. We show that BPIFB3 is associated with the endoplasmic reticulum (ER), and its silencing by RNA interference enhances basal levels of autophagy and promotes increased autophagy during CVB replication. Conversely, overexpression of BPIFB3 inhibits CVB replication, dramatically alters the morphology of LC3B-positive vesicles, and suppresses autophagy in response to rapamaycin. In addition, we found that, whereas silencing of core autophagy components associated with the initiation of APs in control cells suppressed CVB replication, silencing of these same components had no effect on CVB-induced autophagy or viral replication in cells transfected with BPIFB3 small interfering RNA. Based on these results, taken together, this study reports on a previously uncharacterized regulator of enterovirus infection that controls replication through a noncanonical pathway independent from the core autophagy initiation machinery.

Coxsackievirus B (CVB) infections are commonly associated with dilated cardiomyopathy, a condition that accounts for nearly half of all heart transplants annually. During infection, CVB co-opts a cellular pathway, termed autophagy, to provide the membranes necessary for its replication. Autophagy is an evolutionarily conserved process by which cells ingest damaged organelles as a means of maintaining cell homeostasis. Here, we report on a novel regulator of autophagy, bactericidal/permeability-increasing protein (BPI) fold-containing family B, member 3 (BPIFB3), whose expression functions to restrict CVB replication by suppressing key steps in the authophagic process. We show that loss of BPIFB3 expression greatly enhances CVB replication while having no effect on replication of poliovirus, a closely related virus. Our results thus identify a novel host cell therapeutic target whose function could be targeted to alter CVB replication.

Autophagy, viruses, and intestinal immunity

PURPOSE OF REVIEW

To highlight recent findings that identify an essential role for the cellular degradative pathway of autophagy in governing a balanced response to intestinal pathogens and commensals.

RECENT FINDINGS

Following the genetic association of autophagy with inflammatory bowel disease susceptibility, increasing evidence indicates that this pathway functions in various epithelial lineages to support the intestinal barrier. New studies are also revealing that autophagy proteins dictate the quality and magnitude of immune responses. Mouse models, in particular, suggest that autophagy and inflammatory bowel disease susceptibility genes regulate inflammatory responses to viruses, a finding that coincides with an increasing appreciation that viruses have intricate interactions with the host and the microbiota beyond the obvious host-pathogen relationship.

SUMMARY

Autophagy and other immunological or stress response pathways intersect in mucosal immunity to dictate the response to pathogenic and commensal agents. The development of novel treatment strategies, as well as prognostic and diagnostic tools for gastrointestinal disorders, will be greatly facilitated by a deeper understanding of these interactions at the cell type and microbe-specific manner, which includes less appreciated components of the microbiota, such as eukaryotic and prokaryotic viruses.

Coronavirus membrane-associated papain-like proteases induce autophagy through interacting with Beclin1 to negatively regulate antiviral innate immunity

Autophagy plays important roles in modulating viral replication and antiviral immune response. Coronavirus infection is associated with the autophagic process, however, little is known about the mechanisms of autophagy induction and its contribution to coronavirus regulation of host innate responses. Here, we show that the membrane-associated papain-like protease PLP2 (PLP2-TM) of coronaviruses acts as a novel autophagy-inducing protein. Intriguingly, PLP2-TM induces incomplete autophagy process by increasing the accumulation of autophagosomes but blocking the fusion of autophagosomes with lysosomes. Furthermore, PLP2-TM interacts with the key autophagy regulators, LC3 and Beclin1, and promotes Beclin1 interaction with STING, the key regulator for antiviral IFN signaling. Finally, knockdown of Beclin1 partially reverses PLP2-TM's inhibitory effect on innate immunity which resulting in decreased coronavirus replication. These results suggested that coronavirus papain-like protease induces incomplete autophagy by interacting with Beclin1, which in turn modulates coronavirus replication and antiviral innate immunity.

Poliovirus-induced changes in cellular membranes throughout infection

The membrane landscape of a cell often changes drastically upon infection by a virus. In the case of the well-studied positive strand RNA virus poliovirus, the short infection cycle induces vesicles and tubular structures early in infection, and double-membraned vesicles late in infection. In this review, the current understanding of membrane changes in a PV-infected cell, the host and viral factors that facilitate these changes, and how these changes may promote virus replication will be discussed. Host factors involved in membrane rearrangement during infection include components of the COPI and COPII secretory pathways, lipid kinases, and the autophagy pathway. The roles of cellular membranes include acting as a scaffold for the RNA replication complex and roles in exit of mature virus. Finally, recent studies suggesting that not all picornaviruses are truly 'non-enveloped' are discussed in the context of the field, raising the possibility that cell-derived membranes play a role in delivering poliovirus particles to the extracellular space.

Topology and biological function of enterovirus non-structural protein 2B as a member of the viroporin family

Viroporins are a group of transmembrane proteins with low molecular weight that are encoded by many animal viruses. Generally, viroporins are composed of 50–120 amino acid residues and possess a minimum of one hydrophobic region that interacts with the lipid bilayer and leads to dispersion. Viroporins are involved in destroying the morphology of host cells and disturbing their biological functions to complete the life cycle of the virus. The 2B proteins encoded by enteroviruses, which belong to the family Picornaviridae, can form transmembrane pores by oligomerization, increase the permeability of plasma membranes, disturb the homeostasis of calcium in cells, induce apoptosis, and cause autophagy; these abilities are shared among viroporins. The present paper introduces the structure and biological characteristics of various 2B proteins encoded by enteroviruses of the family Picornaviridae and may provide a novel idea for developing antiviral drugs.

AUF1 is recruited to the stress granules induced by coxsackievirus B3

Stress granules (SGs) are cytoplasmic granules that are formed in cells when stress occurs. In this study, we found that SGs formed in cells infected with coxsackievirus B3 (CVB3), evidenced with the co-localization of some accepted SG markers in the viral infection-induced granules. We further discovered that adenosine-uridine (AU)-rich element RNA binding factor 1 (AUF1), which can bind to mRNAs and regulate their translation, was recruited to the SGs in response to high dose of CVB3 by detecting the co-localization of AUF1 with SG markers. Similar results were also observed in the enterovirus 71 (EV71)-infected cells. Finally, we demonstrated that AUF1 was also recruited to arsenite-induced SGs, suggesting that the recruitment of AUF1 to SG is not a specific response to viral infection. In summary, our data indicate that both CVB3 and EV71 infections can induce SG formation, and AUF1 is a novel SG component upon the viral infections. Our findings may shed light on understanding the picornavirus-host interaction.

Enterovirus 71-induced autophagy increases viral replication and pathogenesis in a suckling mouse model

Background

We previously reported that Enterovirus 71 (EV71) infection activates autophagy, which promotes viral replication both in vitro and in vivo. In the present study we further investigated whether EV71 infection of neuronal SK-N-SH cells induces an autophagic flux. Furthermore, the effects of autophagy on EV71-related pathogenesis and viral load were evaluated after intracranial inoculation of mouse-adapted EV71 (MP4 strain) into 6-day-old ICR suckling mice.

Results

We demonstrated that in EV71-infected SK-N-SH cells, EV71 structural protein VP1 and nonstructural protein 2C co-localized with LC3 and mannose-6-phosphate receptor (MPR, endosome marker) proteins by immunofluorescence staining, indicating amphisome formation. Together with amphisome formation, EV71 induced an autophagic flux, which could be blocked by NH₄Cl (inhibitor of acidification) and vinblastine (inhibitor of fusion), as demonstrated by Western blotting. Suckling mice intracranially inoculated with EV71 showed EV71 VP1 protein expression (representing EV71 infection) in the cerebellum, medulla, and pons by immunohistochemical staining. Accompanied with these infected brain tissues, increased expression of LC3-II protein as well as formation of LC3 aggregates, autophagosomes and amphisomes were detected. Amphisome formation, which was confirmed by colocalization of EV71-VP1 protein or LC3 puncta and the endosome marker protein MPR. Thus, EV71-infected suckling mice (similar to EV71-infected SK-N-SH cells) also show an autophagic flux. The physiopathological parameters of EV71-MP4 infected mice, including body weight loss, disease symptoms, and mortality were increased compared to those of the uninfected mice. We further blocked EV71-induced autophagy with the inhibitor 3-methyladenine (3-MA), which attenuated the disease symptoms and decreased the viral load in the brain tissues of the infected mice.

Conclusions

In this study, we reveal that EV71 infection of suckling mice induces an amphisome formation accompanied with the autophagic flux in the brain tissues. Autophagy induced by EV71 promotes viral replication and EV71-related pathogenesis.

Interplay of autophagy and apoptosis during murine cytomegalovirus infection of RPE cells

PURPOSE

Previous studies have demonstrated that autophagy is involved in the pathogenesis of human cytomegalovirus (HCMV) infection. However, whether autophagy is regulated by murine cytomegalovirus (MCMV) infection has not yet been investigated. The purpose of these studies was to determine how autophagy is affected by MCMV infection of the retinal pigment epithelial (RPE) cells and whether there is a functional relationship between autophagy and apoptosis; and if so, how regulation of autophagy impacts apoptosis.

METHODS

RPE cells were isolated from C57BL/6 mice and infected with MCMV K181. The cells were cultured in medium containing rapamycin, chloroquine, or ammonium chloride. Green fluorescent protein-light chain 3 (GFP-LC3) plasmid was transfected to RPE cells, and the GFP-LC3 positive puncta were counted. Electron microscopic (EM) images were taken to visualize the structure of the autophagic vacuoles. Western blot was performed to detect the expression of related proteins. Trypan blue exclusion assay was used to measure the percentage of viable cells.

RESULTS

Although the LC3B-II levels consistently increased during MCMV infection of RPE cells, administration of chloroquine or ammonium chloride increased LC3B-II expression only at the early stage of infection (6 h post-inoculation [p.i.] and 12 h p.i.), not at or after 24 h p.i. The punctate autophagic vacuoles in the GFP-LC3 transfected RPE cells were counted using light microscopy or by EM examination, the number of autophagic vacuoles was significantly increased in the MCMV-infected RPE cells compared to the uninfected controls. Compared to untreated MCMV-infected control cells, rapamycin treatment resulted in a significant decrease in the cleaved caspase 3 levels as well as a significant decrease in the ratio of phosphorylated mammalian target of rapamycin (mTOR) to total mTOR and in the ratio of phosphorylated P70S6K to total P70S6K. In contrast, chloroquine treatment resulted in a significant increase in the cleaved caspase 3 levels in the MCMV-infected RPE cells.

CONCLUSIONS

Autophagic vacuole accumulation was detected during MCMV infection of RPE cells. In contrast, autophagic flux was greatly decreased at or after 24 h p.i. The results suggest that MCMV might have a strategy for inhibiting or blocking autophagy activity by targeting a later autophagy process, such as the formation of autolysosomes or degradation of their content. Our data also suggest that there is a functional relationship between autophagy and apoptosis, which plays an important role during MCMV infection of the RPE.

ER stress, autophagy, and RNA viruses

Endoplasmic reticulum (ER) stress is a general term for representing the pathway by which various stimuli affect ER functions. ER stress induces the evolutionarily conserved signaling pathways, called the unfolded protein response (UPR), which compromises the stimulus and then determines whether the cell survives or dies. In recent years, ongoing research has suggested that these pathways may be linked to the autophagic response, which plays a key role in the cell's response to various stressors. Autophagy performs a self-digestion function, and its activation protects cells against certain pathogens. However, the link between the UPR and autophagy may be more complicated. These two systems may act dependently, or the induction of one system may interfere with the other. Experimental studies have found that different viruses modulate these mechanisms to allow them to escape the host immune response or, worse, to exploit the host's defense to their advantage; thus, this topic is a critical area in antiviral research. In this review, we summarize the current knowledge about how RNA viruses, including influenza virus, poliovirus, coxsackievirus, enterovirus 71, Japanese encephalitis virus, hepatitis C virus, and dengue virus, regulate these processes. We also discuss recent discoveries and how these will produce novel strategies for antiviral treatment.

Membranous replication factories induced by plus-strand RNA viruses

In this review, we summarize the current knowledge about the membranous replication factories of members of plus-strand (+) RNA viruses. We discuss primarily the architecture of these complex membrane rearrangements, because this topic emerged in the last few years as electron tomography has become more widely available. A general denominator is that two “morphotypes” of membrane alterations can be found that are exemplified by flaviviruses and hepaciviruses: membrane invaginations towards the lumen of the endoplasmatic reticulum (ER) and double membrane vesicles, representing extrusions also originating from the ER, respectively. We hypothesize that either morphotype might reflect common pathways and principles that are used by these viruses to form their membranous replication compartments.