Porcine reproductive and respiratory syndrome virus infection activates NOD2-RIP2 signal pathway in MARC-145 cells

Grouper RIP2 inhibits Singapore grouper iridovirus infection by modulating ASC-caspase-1 interaction

Introduction

Receptor interacting protein 2 (RIP2), serves as a vital sensor of cell stress, is able to respond to cell survival or inflammation, and is involved in antiviral pathways. However, studies on the property of RIP2 in viral infections in fish have not been reported.

Methods

In this paper, we cloned and characterized RIP2 homolog from orange-spotted grouper (Epinephelus coioides) (EcRIP2) and further discussed the relevance of EcRIP2 to EcASC, comparing the influences of EcRIP2 and EcASC on the modulation of inflammatory factors and the NF-κB activation to reveal the mechanism of EcRIP2 in fish DNA virus infection.

Results

Encoded a 602 amino acid protein, EcRIP2 contained two structural domains: S-TKc and CARD. Subcellular localization signified that EcRIP2 existed in cytoplasmic filaments and dot aggregation patterns. After SGIV infection, the EcRIP2 filaments aggregated into larger clusters near the nucleus. The infection of SGIV could notably up-regulate the transcription level of the EcRIP2 gene compared with lipopolysaccharide (LPS) and red grouper nerve necrosis virus (RGNNV). Overexpression of EcRIP2 impeded SGIV replication. The elevated expression levels of inflammatory cytokines induced by SGIV were remarkably hindered by EcRIP2 treatment in a concentration-dependent manner. In contrast, EcASC treatment could up-regulate SGIV-induced cytokine expression in the presence of EcCaspase-1. Enhancing amounts of EcRIP2 could overcome the down regulatory effect of EcASC on NF-κB. Nevertheless, increasing doses of EcASC failed to restrain the NF-κB activation in the existence of EcRIP2. Subsequently, it was validated by a co-immunoprecipitation assay that EcRIP2 dose-dependently competed with EcASC binding to EcCaspase-1. With increasing time to SGIV infection, EcCaspase-1 gradually combined with more EcRIP2 than EcASC.

Discussion

Collectively, this paper highlighted that EcRIP2 may impede SGIV-induced hyperinflammation by competing with EcASC for binding EcCaspase-1, thereby suppressing viral replication of SGIV. Our work supplies novel viewpoints into the modulatory mechanism of RIP2-associated pathway and offers a novel view of RIP2-mediated fish diseases.

Transcriptome alterations in chicken HD11 cells with steady knockdown and overexpression of RIPK2 gene

Receptor interacting protein kinase 2 (RIPK2) is involved in a variety of signaling pathway to produce a series of inflammatory cytokines in response to a diverse of bacterial, viral and protozoal pathogens. However, the underlying regulating of RIPK2 remain unknown. Transcriptome alterations in chicken HD11 cells following RIPK2 overexpression or silencing by shRNA were analyzed by next-generation sequencing. Both overexpression and knockdown of the RIPK2 gene caused wide-spread changes in gene expression in chicken HD11 cells. Differentially expressed genes (DEGs) caused by altered RIPK2 gene expression were associated with multiple biological processes linked with biological regulation, response to stimulus, cell communication, and signal transduction etc. KEGG analysis revealed that many of the DEGs were enriched in VEGF signaling pathway, ECM-receptor interaction, Focal adhesion, TGF-beta signaling pathway etc. Moreover, we show that initiation genes, TGFB1 and TGFB3, in the TGF-beta signaling pathway are biological targets regulated by RIPK2 in chicken HD11 cells. This is the first transcriptome-wide study in which RIPK2-regulated genes in chicken cells have been screened. Our findings elucidate the molecular events associated with RIPK2 in chicken HD11 cells.

Interactions of Autophagy and the Immune System in Health and Diseases

Autophagy is a highly conserved process that utilizes lysosomes to selectively degrade a variety of intracellular cargo, thus providing quality control over cellular components and maintaining cellular regulatory functions. Autophagy is triggered by multiple stimuli ranging from nutrient starvation to microbial infection. Autophagy extensively shapes and modulates the inflammatory response, the concerted action of immune cells, and secreted mediators aimed to eradicate a microbial infection or to heal sterile tissue damage. Here, we first review how autophagy affects innate immune signaling, cell-autonomous immune defense, and adaptive immunity. Then, we discuss the role of non-canonical autophagy in microbial infections and inflammation. Finally, we review how crosstalk between autophagy and inflammation influences infectious, metabolic, and autoimmune disorders.

Mycoplasma hyopneumoniae Infection Activates the NOD1 Signaling Pathway to Modulate Inflammation

Mycoplasma hyopneumoniae is a highly contagious pathogen causing porcine enzootic pneumonia, which elicits prolonged inflammatory response modulated by pattern recognition receptors (PRRs). Although significant advances have been achieved in understanding the Toll-Like receptors that recognize M. hyopneumoniae, the role of nucleotide-binding oligomerization domain 1 (NOD1) in M. hyopneumoniae infected cells remains poorly understood. This study revealed that M. hyopneumoniae activates the NOD1-RIP2 pathway and is co-localized with host NOD1 during infection. siRNA knockdown of NOD1 significantly impaired the TRIF and MYD88 pathway and blocked the activation of TNF-α. In contrast, NOD1 overexpression significantly suppressed M. hyopneumoniae proliferation. Furthermore, we for the first time investigated the interaction between M. hyopneumoniae mhp390 and NOD1 receptor, and the results suggested that mhp390 and NOD1 are possibly involved in the recognition of M. hyopneumoniae. These findings may improve our understanding of the interaction between PRRs and M. hyopneumoniae and the function of NOD1 in host defense against M. hyopneumoniae infection.

Nucleotide-Binding Oligomerization Domain 1 (NOD1) Positively Regulates Neuroinflammation during Japanese Encephalitis Virus Infection

Japanese encephalitis virus (JEV) is a neurotropic flavivirus that invades the central nervous system and causes neuroinflammation and extensive neuronal cell death. Nucleotide-binding oligomerization domain 1 (NOD1) is a type of pattern recognition receptor that plays a regulatory role in both bacterial and nonbacterial infections. However, the role of NOD1 in JEV-induced neuroinflammation remains undisclosed. In this study, we evaluated the effect of NOD1 activation on the progression of JEV-induced neuroinflammation using a human astrocytic cell line and NOD1 knockout mice. The results showed that JEV infection upregulated the mRNA and protein expression of NOD1, ultimately leading to an enhanced neuroinflammatory response in vivo and in vitro. Inhibition of NOD1 in cultured cells or mice significantly abrogated the inflammatory response triggered by JEV infection. Moreover, compared to the wild-type mice, the NOD1 knockout mice showed resistance to JEV infection. Mechanistically, the NOD1-mediated neuroinflammatory response was found to be associated with increased expression or activation/phosphorylation of downstream receptor-interacting protein 2 (RIPK2), mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase (ERK), Jun N-terminal protein kinase (JNK), and NF-κB signaling molecules. Thus, NOD1 targeting could be a therapeutic approach to treat Japanese encephalitis. IMPORTANCE Neuroinflammation is the main pathological manifestation of Japanese encephalitis (JE) and the most important factor leading to morbidity and death in humans and animals infected by JEV. An in-depth understanding of the basic mechanisms of neuroinflammation will contribute to research on JE treatment. This study proved that JEV infection can activate the NOD1-RIPK2 signal cascade to induce neuroinflammation through the proven downstream MAPK, ERK, JNK, and NF-κB signal pathway. Thus, our study unveiled NOD1 as a potential target for therapeutic intervention for JE.

Functions of Viroporins in the Viral Life Cycle and Their Regulation of Host Cell Responses

Viroporins are virally encoded transmembrane proteins that are essential for viral pathogenicity and can participate in various stages of the viral life cycle, thereby promoting viral proliferation. Viroporins have multifaceted effects on host cell biological functions, including altering cell membrane permeability, triggering inflammasome formation, inducing apoptosis and autophagy, and evading immune responses, thereby ensuring that the virus completes its life cycle. Viroporins are also virulence factors, and their complete or partial deletion often reduces virion release and reduces viral pathogenicity, highlighting the important role of these proteins in the viral life cycle. Thus, viroporins represent a common drug-protein target for inhibiting drugs and the development of antiviral therapies. This article reviews current studies on the functions of viroporins in the viral life cycle and their regulation of host cell responses, with the aim of improving the understanding of this growing family of viral proteins.

TRIM26-mediated degradation of nucleocapsid protein limits porcine reproductive and respiratory syndrome virus-2 infection

Porcine reproductive and respiratory syndrome (PRRS), caused by PRRSV, has ranked among the most economically important veterinary infectious diseases globally. Recently, tripartite motif (TRIMs) family members have arisen as novel restriction factors in antiviral immunity. Noteworthy, TRIM26 was reported as a binding partner of IRF3, TBK1, TAB1, and NEMO, yet its role in virus infection remains controversial. Herein, we showed that TRIM26 bound N protein by the C-terminal PRY/SPRY domain. Moreover, ectopic expression of TRIM26 impaired PRRSV replication and induced degradation of N protein. The anti-PRRSV activity was independent of the nuclear localization signal (NLS). Instead, deletion of the RING domain, or the PRY/SPRY portion, abrogated the antiviral function. Finally, siRNA depletion of TRIM26 resulted in enhanced production of viral RNA and virus yield in porcine alveolar macrophages (PAMs) after PRRSV infection. Overexpression of an RNAi-resistant TRIM26 rescue-plasmid led to the acquisition of PRRSV restriction in TRIM26-knockdown cells. Together, these data add TRIM26 as a potential target for drug design against PRRSV.

Transcriptome sequencing analysis of porcine alveolar macrophages infected with PRRSV strains to elucidate virus pathogenicity and immune evasion strategies

Highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) causes a serious disease to the swine industry worldwide. To understand the mechanisms of HP-PRRSV infection, RNA-seq-based transcriptome analyses were performed on porcine alveolar macrophages (PAMs) infected with a HP-PRRSV strain (TJ), a less virulent strain of a classical lineage (CH-1a), and a vaccine strain TJM-F92. Gene ontology, Kyoto Encyclopedia of Genes and Genomes analyses indicate that TJM-F92 led to significant up-regulation of gene expression for proteins associated with membrane-bound organelles. The differentially expressed genes of HP-PRRSV TJ-infected PAM cells were up-regulated in the special G-protein coupled receptor. The six cytokines were tested by real time Reverse Transcription-Polymerase Chain Reaction (RT-PCR). The relative expression levels showed the same trend of expression difference. Significant up-regulation of TMEM173 plays an important role in the cytosolic DNA-sensing pathway and the RIG-I-like receptor signaling pathway in TJM-F92 infected PAM cells. These data provide new insight into PRRSV pathogenicity and immune evasion strategies.

Immunologic mechanism of fungal keratitis

Fungal keratitis (FK) is a refractory disease that poses a serious threat to vision, with common risk factors like eye trauma, contact lens wearing, topical corticosteroids and antibiotic abuse. Nowadays, topical and systemic anti-fungal drugs and ocular surgeries are still the main therapeutic modalities. However, the pathogenesis of FK, especially the immunologic mechanism within it, has not yet been deeply clarified. A better understanding of the pathogenesis of FK is imperative for more effective therapies and prognosis. Meanwhile, the immune protection strategies are also urgently required to manage FK. This review highlights recent advances in the immunologic mechanism in the pathogenesis of FK, in hope of providing valuable reference information for more effective anti-fungal treatment.

RICK regulates the odontogenic differentiation of dental pulp stem cells through activation of TNF-α via the ERK and not through NF-κB signaling pathway

Dental pulp stem cells (DPSCs) are capable of both self-renewal and multilineage differentiation, which play a positive role in dentinogenesis. Studies have shown that tumor necrosis factor-α (TNF-α) is involved in the differentiation of DPSCs under pro-inflammatory stimuli, but the mechanism of action of TNF-α is unknown. Rip-like interacting caspase-like apoptosis-regulatory protein kinase (RICK) is a biomarker of an early inflammatory response that plays a key role in modulating cell differentiation, but the role of RICK in DPSCs is still unclear. In this study, we identified that RICK regulates TNF-α-mediated odontogenic differentiation of DPSCs via the ERK signaling pathway. The expression of the biomarkers of odontogenic differentiation dental matrix protein-1 (DMP-1), dentin sialophosphoprotein (DSPP), biomarkers of odontogenic differentiation, increased in low concentration (1-10 ng/ml) of TNF-α and decreased in high concentration (50-100 ng/ml). Odontogenic differentiation increased over time in the odontogenic differentiation medium. In the presence of 10 ng/L TNF-α, the expression of RICK increased gradually over time, along with odontogenic differentiation. Genetic silencing of RICK expression reduced the expression of odontogenic markers DMP-1 and DSPP. The ERK, but not the NF-κB signaling pathway, was activated during the odontogenic differentiation of DPSCs. ERK signaling modulators decreased when RICK expression was inhibited. PD98059, an ERK inhibitor, blocked the odontogenic differentiation of DPSCs induced by TNF-α. These results provide a further theoretical and experimental basis for the potential use of RICK in targeted therapy for dentin regeneration.

Foot-and-Mouth Disease Virus Inhibits RIP2 Protein Expression to Promote Viral Replication

Receptors interaction protein 2 (RIP2) is a specific adaptor molecule in the downstream of NOD2. The role of RIP2 during foot-and-mouth disease virus (FMDV) infection remains unknown. Here, our results showed that RIP2 inhibited FMDV replication and played an important role in the activation of IFN-β and NF-ĸB signal pathways during FMDV infection. FMDV infection triggered RIP2 transcription, while it reduced the expression of RIP2 protein. Detailed analysis showed that FMDV 2B, 2C, 3C^pro, and L^pro proteins were responsible for inducing the reduction of RIP2 protein. 3C^pro and L^pro are viral proteinases that can induce the cleavage or reduction of many host proteins and block host protein synthesis. The carboxyl terminal 105-114 and 135-144 regions of 2B were essential for reduction of RIP2. Our results also showed that the N terminal 1-61 region of 2C were essential for the reduction of RIP2. The 2C-induced reduction of RIP2 was dependent on inducing the reduction of poly(A)-binding protein 1 (PABPC1). The interaction between RIP2 and 2C was observed in the context of viral infection, and the residues 1-61 were required for the interaction. These data clarify novel mechanisms of reduction of RIP2 mediated by FMDV.

Molecular and Cellular Mechanisms for PRRSV Pathogenesis and Host Response to Infection

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PRRSV has evolved to arm with various strategies to modify host antiviral response.

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Viral modulation of homeostatic cellular processes provides favorable conditions for PRRSV survival during infection.

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PRRSV modulation of cellular processes includes pathways for interferons, apoptosis, microRNAs, cytokines, autophagy, and viral genome recombination.

Porcine reproductive and respiratory syndrome virus (PRRSV) has caused tremendous amounts of economic losses to the swine industry for more than three decades, but its control is still unsatisfactory. A significant amount of information is available for host cell-virus interactions during infection, and it is evident that PRRSV has evolved to equip various strategies to disrupt the host antiviral system and provide favorable conditions for survival. The current study reviews viral strategies for modulations of cellular processes including innate immunity, apoptosis, microRNAs, inflammatory cytokines, and other cellular pathways.

Nuclear localization signal in TRIM22 is essential for inhibition of type 2 porcine reproductive and respiratory syndrome virus replication in MARC-145 cells

Porcine reproductive and respiratory syndrome virus (PRRSV) infection causes one of the most economically important swine diseases worldwide. Tripartite motif-containing 22 (TRIM22), a TRIM family protein, has been identified as a crucial restriction factor that inhibits a group of human viruses. Currently, the role of cellular TRIM22 in PRRSV infection remains unclear. In the present study, we analyzed the effect of TRIM22 on PRRSV replication in vitro and explored the underlying mechanism. Ectopic expression of TRIM22 impaired the viral replication, while TRIM22-RNAi favored the replication of PRRSV in MARC-145 cells. Additionally, we observed that TRIM22 deletion SPRY domain or Nuclear localization signal (NLS) losses the ability to inhibit PRRSV replication. Finally, Co-IP analysis identified that TRIM22 interacts with PRRSV nucleocapsid (N) protein through the SPRY domain, while the NLS2 motif of N protein is involved in interaction with TRIM22. Although the concentration of PRRSV N protein was not altered in the presence of TRIM22, the abundance of N proteins from simian hemorrhagic fever virus (SHFV), equine arteritis virus (EAV), and murine lactate dehydrogenase-elevating virus (LDV) diminished considerably with increasing TRIM22 expression. Together, our findings uncover a previously unrecognized role for TRIM22 and extend the antiviral effects of TRIM22 to arteriviruses.

MicroRNA transcriptome analysis of poly I:C-stimulated and PRRSV-infected porcine alveolar macrophages

Porcine reproductive and respiratory syndrome virus (PRRSV) causes severe reproductive failure in sows, respiratory diseases, and high mortality in piglets, which results in serious economic losses to the swine industry worldwide. Previous studies have described that PRRSV could suppress the host immune system and had antiapoptotic activity in its initial phase of infection. Polyinosinic-polycytidylic acid (poly I:C), a synthesized analogue of viral double-strand RNA, activates innate immunity responses and induces apoptosis in cells. Therefore, we performed miRNA transcriptome analysis of poly I:C-stimulated and PRRSV-infected porcine alveolar macrophages (PAMs) using deep sequencing technology, to compare the different miRNA profiles between the statuses of innate immune activation and inactivation. After sequencing, 267 known mature miRNAs and 64 novel miRNAs were observed in PAMs, and a total of 197 miRNAs were significantly differently expressed in poly I:C-stimulated PAMs, compared with mock control cells. Thirty-three of them were also significantly alerted in PRRSV-infected PAMs. This indicated that PRRSV only slightly alerted the miRNA expression profile of host cells compared with poly I:C-stimulated PAMs, which confirmed that PRRSV could suppress host innate immune responses during the early stages of infection. Among the differentially expressed miRNAs, we found that ssc-miR-27b-3p could significantly inhibit PRRSV RNA and protein replication in MARC-145 cells and PAMs. Its antiviral mechanism needs further research in the future.

Nucleotide-binding oligomerization domain-like receptor X1 restricts porcine reproductive and respiratory syndrome virus-2 replication by interacting with viral Nsp9

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PRRSV infection up-regulates NLRX1 expression.

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NLRX1 impairs PRRSV replication.

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NLRX1 suppresses the synthesis of viral subgenomic RNAs.

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NLRX1 interacts and colocalizes with the Nsp9 of PRRSV.

Porcine reproductive and respiratory syndrome virus (PRRSV) causes one of the most economically important diseases of swine worldwide. Current antiviral strategies provide only limited protection. Nucleotide-binding oligomerization domain-like receptor (NLR) X1 is unique among NLR proteins in its functions as a pro-viral or antiviral factor to different viral infections. To date, the impact of NLRX1 on PRRSV infection remains unclear. In this study, we found that PRRSV infection promoted the expression of NLRX1 gene. In turn, ectopic expression of NLRX1 inhibited PRRSV replication in Marc-145 cells, whereas knockdown of NLRX1 enhanced PRRSV propagation in porcine alveolar macrophages (PAMs). Mechanistically, NLRX1 was revealed to impair intracellular viral subgenomic RNAs accumulation. Finally, Mutagenic analyses indicated that the LRR (leucine-rich repeats) domain of NLRX1 interacted with PRRSV Nonstructural Protein 9 (Nsp9) RdRp (RNA-dependent RNA Polymerase) domain and was necessary for antiviral activity. Thus, our study establishes the role of NLRX1 as a new host restriction factor in PRRSV infection.

Foot-and-Mouth Disease Virus Antagonizes NOD2-Mediated Antiviral Effects by Inhibiting NOD2 Protein Expression

The role of nucleotide-binding oligomerization domain 2 (NOD2) in foot-and-mouth disease virus (FMDV)-infected cells remains unknown. Here, we showed that FMDV infection activated NOD2-mediated beta interferon (IFN-β) and nuclear factor-κB (NF-ĸB) signaling pathways. NOD2 inhibited FMDV replication in the infected cells. FMDV infection triggered NOD2 transcription, while it reduced the abundance of NOD2 protein. Our results revealed that FMDV 2B, 2C, and 3C proteinase (3C^pro) were responsible for the decrease in NOD2 protein levels. 3C^pro is a viral proteinase that can cleave multiple host proteins and limit protein synthesis. Our previous studies determined that FMDV 2B suppressed protein expression of RIG-I and LGP2. Here, we found that 3C^pro and 2B also decreased NOD2 expression. However, this is the first report that 2C induced the reduction of NOD2 protein levels. We determined that both 2B- and 2C-induced decreases in NOD2 were independent of the cleavage of host eukaryotic translation initiation factor 4 gamma (eIF4G), induction of cellular apoptosis, or proteasome, lysosome, and caspase pathways. The interactions between NOD2 and 2B or 2C were observed in the context of viral infection. The carboxyl-terminal amino acids 105 to 114 and 135 to 144 of 2B were essential for the reduction of NOD2, while the residues 105 to 114 were required for the interaction. Amino acids 116 to 260 of the carboxyl terminus of 2C were essential for the interaction, while truncated 2C mutants did not reduce NOD2. These data suggested novel antagonistic mechanisms of FMDV that were mediated by 2B, 2C, and 3C^pro proteins.IMPORTANCE NOD2 was identified as a cytoplasmic viral pattern recognition receptor in 2009. Subsequently, many viruses were reported to activate NOD2-mediated signaling pathways. This study demonstrated that FMDV infection activated NOD2-mediated IFN-β and NF-ĸB signaling pathways. Host cells have developed multiple strategies against viral infection; however, viruses have evolved many strategies to escape host defenses. FMDV has evolved multiple mechanisms to inhibit host type I IFN production. Here, we showed that NOD2 suppressed FMDV replication during viral infection. FMDV 2B, 2C, and 3C^pro decreased NOD2 protein expression by different mechanisms to promote viral replication. This study provided new insight into the immune evasion mechanisms mediated by FMDV and identified 2B, 2C, and 3C^pro as antagonistic factors for FMDV to evade host antiviral responses.

Signaling pathways involved in regulating apoptosis induction in host cells upon PRRSV infection

Porcine reproductive and respiratory syndrome virus (PRRSV) is the etiologic agent of porcine reproductive and respiratory syndrome (PRRS), a devastating disease of swine that poses a serious threat to the swine industry worldwide. The induction of apoptosis in host cells is suggested to be the key cellular mechanism that contributes to the pathogenesis of PRRS. Various signaling pathways have been identified to be involved in regulating PRRSV-induced apoptosis. In this review, we summarize the potential signaling pathways that contribute to PRRSV-induced apoptosis, and propose the issues that need to be addressed in future studies for a better understanding of the molecular basis underlying the pathogenesis of PRRS.

RIP2 Is a Critical Regulator for NLRs Signaling and MHC Antigen Presentation but Not for MAPK and PI3K/Akt Pathways

RIP2 is an adaptor protein which is essential for the activation of NF-κB and NOD1- and NOD2-dependent signaling. Although NOD-RIP2 axis conservatively existed in the teleost, the function of RIP2 was only reported in zebrafish, goldfish, and rainbow trout in vitro. Very little is known about the role and mechanisms of piscine NOD-RIP2 axis in vivo. Our previous study showed the protective role of zebrafish NOD1 in larval survival through CD44a-mediated activation of PI3K-Akt signaling. In this study, we examined whether RIP2 was required for larval survival with or without pathogen infection, and determined the signaling pathways modulated by RIP2. Based on our previous report and the present study, our data demonstrated that NOD1-RIP2 axis was important for larval survival in the early ontogenesis. Similar to NOD1, RIP2 deficiency significantly affected immune system processes. The significantly enriched pathways were mainly involved in immune system, such as “Antigen processing and presentation” and “NOD-like receptor signaling pathway” and so on. Furthermore, both transcriptome analysis and qRT-PCR revealed that RIP2 was a critical regulator for expression of NLRs (NOD-like receptors) and those genes involved in MHC antigen presentation. Different from NOD1, the present study showed that NOD1, but not RIP2 deficiency significantly impaired protein levels of MAPK pathways. Although RIP2 deficiency also significantly impaired the expression of CD44a, the downstream signaling of CD44a-Lck-PI3K-Akt pathway remained unchanged. Collectively, our works highlight the similarity and discrepancy of NOD1 and RIP2 in the regulation of immune signaling pathways in the zebrafish early ontogenesis, and confirm the crucial role of RIP2 in NLRs signaling and MHC antigen presentation, but not for MAPK and PI3K/Akt pathways.

Porcine reproductive and respiratory syndrome virus triggers mitochondrial fission and mitophagy to attenuate apoptosis

Porcine reproductive and respiratory syndrome virus (PRRSV) causes acute mitochondrial dysfunction by elevating the level of reactive oxygen species. Mitochondrial dynamics and mitophagy are essential for the maintenance of mitochondrial homeostasis. Here we show that PRRSV infection stimulated mitochondrial fission and mitophagy to attenuate apoptosis in Marc145 cells. PRRSV infection induced the expression of Drp1, enhanced phosphorylation of Drp1 at Ser616 and its subsequent translocation to mitochondria. Furthermore, PRRSV infection increased the expression of PINK1 and Parkin and also stimulated the recruitment of Parkin to mitochondria. In addition, a sensitive dual fluorescence vector expressing mito-mRFP-EGFP targeted mitochondria was employed to observe the complete mitophagy by delivering dysfunctional mitochondria to lysosome for degradation. Interfering the expression of Drp1 and or Parkin suppressed PRRSV replication. More importantly, silencing of Drp1 or Parkin caused significant elevation in apoptotic signaling. These results suggest that PRRSV infection stimulates mitochondrial fission and mitophagy to facilitate virus replication most probably by attenuating apoptosis.

DExD/H-Box Helicase 36 Signaling via Myeloid Differentiation Primary Response Gene 88 Contributes to NF-κB Activation to Type 2 Porcine Reproductive and Respiratory Syndrome Virus Infection

DExD/H-box helicase 36 (DHX36) is known to be an ATP-dependent RNA helicase that unwinds the guanine-quadruplexes DNA or RNA, but emerging data suggest that it also functions as pattern recognition receptor in innate immunity. Porcine reproductive and respiratory syndrome virus (PRRSV) is an Arterivirus that has been devastating the swine industry worldwide. Interstitial pneumonia is considered to be one of the most obvious clinical signs of PRRSV infection, suggesting that the inflammatory response plays an important role in PRRSV pathogenesis. However, whether DHX36 is involved in PRRSV-induced inflammatory cytokine expression remains unclear. In this study, we found that PRRSV infection increased the expression of DHX36. Knockdown of DHX36 and its adaptor myeloid differentiation primary response gene 88 (MyD88) by small-interfering RNA in MARC-145 cells significantly reduced NF-κB activation and pro-inflammatory cytokine expression after PRRSV infection. Further investigation revealed that PRRSV nucleocapsid protein interacted with the N-terminal quadruplex binding domain of DHX36, which in turn augmented nucleocapsid protein-induced NF-κB activation. Taken together, our results suggest that DHX36-MyD88 has a relevant role in the recognition of PRRSV nucleocapsid protein and in the subsequent activation of pro-inflammatory NF-κB pathway.

The Scaffolding Protein IQGAP1 Interacts with NLRC3 and Inhibits Type I IFN Production

Sensing of cytosolic nucleotides is a critical initial step in the elaboration of type I IFN. One of several upstream receptors, cyclic GMP-AMP synthase, binds to cytosolic DNA and generates dicyclic nucleotides that act as secondary messengers. These secondary messengers bind directly to stimulator of IFN genes (STING). STING recruits TNFR-associated NF-κB kinase-binding kinase 1 which acts as a critical node that allows for efficient activation of IFN regulatory factors to drive the antiviral transcriptome. NLRC3 is a recently characterized nucleotide-binding domain, leucine-rich repeat containing protein (NLR) that negatively regulates the type I IFN pathway by inhibiting subcellular redistribution and effective signaling of STING, thus blunting the transcription of type I IFNs. NLRC3 is predominantly expressed in lymphoid and myeloid cells. IQGAP1 was identified as a putative interacting partner of NLRC3 through yeast two-hybrid screening. In this article, we show that IQGAP1 associates with NLRC3 and can disrupt the NLRC3-STING interaction in the cytosol of human epithelial cells. Furthermore, knockdown of IQGAP1 in THP1 and HeLa cells causes significantly more IFN-β production in response to cytosolic nucleic acids. This result phenocopies NLRC3-deficient macrophages and fibroblasts and short hairpin RNA knockdown of NLRC3 in THP1 cells. Our findings suggest that IQGAP1 is a novel regulator of type I IFN production, possibly via interacting with NLRC3 in human monocytic and epithelial cells.

Identification of two mutation sites in spike and envelope proteins mediating optimal cellular infection of porcine epidemic diarrhea virus from different pathways

Entry of the α-coronavirus porcine epidemic diarrhea virus (PEDV) requires specific proteases to activate spike (S) protein for the membrane fusion of the virion to the host cell following receptor binding. Herein, PEDV isolate 85-7 could proliferate and induce cell-cell fusion in a trypsin independent manner on Vero cells, and eight homologous mutation strains were screened by continuous proliferation in the absence of trypsin on Vero cells. According to the whole genome sequence comparative analysis, we identified four major variations located in nonstructural protein 2, S, open reading frame 3, and envelope (E) genes, respectively. Comparative analyses of their genomic variations and proliferation characteristics identified a single mutation within the S2' cleavage site between C30 and C40 mutants: the substitution of conserved arginine (R) by a glycine (G) (R895G). This change resulted in weaker cell-cell fusion, smaller plaque morphology, higher virus titer and serious microfilament condensation. Further analysis confirmed that this mutation was responsible for optimal cell-adaptation, but not the determinant for trypsin-dependent entry of PEDV. Otherwise, a novel variation (16-20 aa deletion and an L25P mutation) in the transmembrane domain of the E protein affected multiple infection processes, including up-regulation of the production of the ER stress indicator GRP78, improving the expression of pro-inflammatory cytokines IL-6 and IL-8, and promoting apoptosis. The results of this study provide a better understanding of the potential mechanisms of viral functional proteins in PEDV replication, infection, and fitness.

Porcine reproductive and respiratory syndrome virus (PRRSV) up-regulates IL-8 expression through TAK-1/JNK/AP-1 pathways

The acute phase of respiratory distress caused by porcine reproductive and respiratory syndrome virus (PRRSV) is likely a consequence of the release of inflammatory cytokines in the lung. IL-8, the main chemokine and activator of neutrophils, might be related to the lung injury upon PRRSV infection. In this study, we showed that PRRSV induced IL-8 expression in vivo and in vitro. Subsequently, we demonstrated that JNK and NF-κB pathways were activated upon PRRSV infection and required for the enhancement of IL-8 expression. We further verified that PRRSV-activated TAK-1 was essential for the activation of JNK and NF-κB pathways and IL-8 expression. Moreover, we revealed an AP-1 binding motif in the cloned porcine IL-8 (pIL-8) promoter, and deletion of this motif abolished the pIL-8 promoter activity. Finally, we found that the JNK-activated AP-1 subunit c-Jun was critical for the up-regulation of IL-8 expression by PRRSV. These data suggest that PRRSV-induced IL-8 production is likely through the TAK-1/JNK/AP-1 pathways.

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PRRSV infection induces IL-8 expression in vitro and in vivo.

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PRRSV up-regulates IL-8 expression through TAK-1/JNK/AP-1 pathways.

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AP-1 element in porcine IL-8 promoter is essential for PRRSV induced IL-8 expression.

Pathogenesis and control of the Chinese highly pathogenic porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) has remained a major threat to the worldwide swine industry ever since its first discovery in the early 1990s. Under the selective pressures in the field, this positive-stranded RNA virus undergoes rapid genetic evolution that eventually leads to emergence in 2006 of the devastating Chinese highly pathogenic PRRSV (HP-PRRSV). The atypical nature of HP-PRRSV has caused colossal economic losses to the swine producers in China and the surrounding countries. In this review, we summarize the recent advances in our understanding of the pathogenesis, evolution and ongoing field practices on the control of this troubling virus in China.

Porcine Reproductive and Respiratory Syndrome Virus nsp1α Inhibits NF-κB Activation by Targeting the Linear Ubiquitin Chain Assembly Complex

Linear ubiquitination, a newly discovered posttranslational modification, is catalyzed by the linear ubiquitin chain assembly complex (LUBAC), which is composed of three subunits: one catalytic subunit HOIP and two accessory molecules, HOIL-1L and SHARPIN. Accumulating evidence suggests that linear ubiquitination plays a crucial role in innate immune signaling and especially in the activation of the NF-κB pathway by conjugating linear polyubiquitin chains to NF-κB essential modulator (NEMO, also called IKKγ), the regulatory subunit of the IKK complex. Porcine reproductive and respiratory syndrome virus (PRRSV), an Arterivirus that has devastated the swine industry worldwide, is an ideal model to study the host's disordered inflammatory responses after viral infection. Here, we found that LUBAC-induced NF-κB and proinflammatory cytokine expression can be inhibited in the early phase of PRRSV infection. Screening the PRRSV-encoded proteins showed that nonstructural protein 1α (nsp1α) suppresses LUBAC-mediated NF-κB activation and its CTE domain is required for the inhibition. Mechanistically, nsp1α binds to HOIP/HOIL-1L and impairs the interaction between HOIP and SHARPIN, thus reducing the LUBAC-dependent linear ubiquitination of NEMO. Moreover, PRRSV infection also blocks LUBAC complex formation and NEMO linear-ubiquitination, the important step for transducing NF-κB signaling. This unexpected finding demonstrates a previously unrecognized role of PRRSV nsp1α in modulating LUBAC signaling and explains an additional mechanism of immune modulation by PRRSV.

IMPORTANCE

Porcine reproductive and respiratory syndrome (PRRS) is one of the most important veterinary infectious diseases in countries with intensive swine industries. PRRS virus (PRRSV) infection usually suppresses proinflammatory cytokine expression in the early stage of infection, whereas it induces an inflammatory storm in the late stage. However, precisely how the virus is capable of doing so remains obscure. In this study, we found that by blocking the interaction of its catalytic subunit HOIP and accessory molecule SHARPIN, PRRSV can suppress NF-κB signal transduction in the early stage of infection. Our findings not only reveal a novel mechanism evolved by PRRSV to regulate inflammatory responses but also highlight the important role of linear ubiquitination modification during virus infection.

Beyond the inflammasome: regulatory NOD-like receptor modulation of the host immune response following virus exposure

A complex interaction exists between elements of the host innate immune system and viral pathogens. It is essential that the host mount a robust immune response during viral infection and effectively resolve inflammation once the pathogen has been eliminated. Members of the nucleotide-binding domain leucine-rich repeat [NBD-LRR; known as NOD-like receptor (NLR)] family of cytosolic pattern-recognition receptors are essential components of these immunological processes and have diverse functions in the host antiviral immune response. NLRs can be subgrouped based on their general function. The inflammasome-forming subgroup of NLRs are the best-characterized family members, and several have been found to modulate the maturation of IL-1β and IL-18 following virus exposure. However, the members of the regulatory NLR subgroups are significantly less characterized. These NLRs uniquely function to modulate signalling pathways initiated by other families of pattern-recognition receptors, such as Toll-like receptors and/or Rig-I-like helicase receptors. Regulatory NLRs that augment pro-inflammatory pathways include nucleotide-binding oligomerization domain-containing protein 1 (NOD1) and NOD2, which have been shown to form a multiprotein complex termed the NODosome that significantly modulates IFN and NF-κB signalling following viral infection. Conversely, a second subgroup of regulatory NLRs functions to negatively regulate inflammation. These inhibitory NLRs include NLRX1, NLRP12 and NLRC3, which have been shown to interact with TRAF molecules and various kinases to modulate diverse cellular processes. Targeting NLR signalling following infection with a virus represents a novel and promising therapeutic strategy. However, significant effort is still required to translate the current understanding of NLR biology into effective therapies.

Interplay of autophagy and apoptosis during PRRSV infection of Marc145 cell

Autophagy and apoptosis play essential roles 'in virus infection. Our study was performed to investigate the interplay between autophagy and apoptosis in PRRSV replication. In our present study, autophagy and apoptosis were induced by PRRSV infection. Viral replication was dampened/attenuated by autophagy deficient and potentiated by apoptosis inhibition. Furthermore, PRRSV replication was restored by apoptosis inhibition in autophagy deficient cells. Taken together, our findings unveil the functional relationship between autophagy and apoptosis during PRRSV replication.

Effect of amino acids residues 323-433 and 628-747 in Nsp2 of representative porcine reproductive and respiratory syndrome virus strains on inflammatory response in vitro

Porcine reproductive and respiratory syndrome virus (PRRSV) is an important pathogen that is responsible for large economic losses in the swine industry worldwide. In PRRSV strains, many genetic variations occur in the central hypervariable region (HV2) of the Nsp2 gene, which encodes non-structural protein 2. For example, PRRSV strains VR2332, Em2007, MN184C, and TJM-F92 contained variations in the Nsp2 sequences and exhibited differing levels of virulence in adult pigs. However, the role of HV2 with respect to PRRSV immunity is unclear. In this study, four recombinant PRRSV strains (rBB/+30aa, rBB/Δ68aa, rBB/Δ111aa, and rBB/Δ120aa) were rescued using a highly pathogenic type 2 PRRSV cDNA clone (pBB). All rescued strains displayed similar growth characteristics to the parental rBB virus in pulmonary alveolar macrophages (PAMs). Expression levels of inflammatory cytokines IL-β, IL-6, and TNF-α were significantly lower, at the mRNA and protein level, for groups infected with rBB/Δ111aa and rBB/Δ120aa than those in the rBB group. Levels of these inflammatory cytokines in the rBB/+30aa and rBB/Δ68aa groups were not significantly different with those in the rBB group. Phosphorylation levels of IκB were decreased to a greater extent in the rBB/Δ111aa and rBB/Δ120aa groups compared with those in the rBB/+30aa, rBB/Δ68aa, and rBB groups. Our results indicate that amino acids 323-433 and 628-747 of Nsp2 failed to exert significant effects on PRRSV replication in PAMs, but modulated the expression of inflammatory cytokines in vitro.

Functional characterization of receptor-interacting serine/threonine kinase 2 (RIP2) of the goldfish (Carassius auratus L.)

We report on the functional characterization of RIP2 of the goldfish. Quantitative expression analysis of goldfish RIP2 revealed the greatest mRNA levels in the spleen, monocytes and splenocytes. We generated a recombinant form of the molecule (rgRIP2) and determined that anti-human RIP2 polyclonal antibody specifically recognized recombinant goldfish RIP2 (rgRIP2). Goldfish RIP2 activity was inhibited by the p38 MAPK pathway inhibitor SB203580. Treatment of goldfish macrophages with LPS, PGN, MDP, Poly I:C, heat-killed and live Mycobacterium marinum, and heat-killed Aeromonas salmonicida differentially changed the expression of RIP2 at both mRNA and protein levels. Co-immunoprecipitation assays indicated that RIP2 interacted with Nod1 and Nod2 receptors in eukaryotic cells. The results of dual luciferase reporter assay revealed that RIP2 over-expression caused the activation of the NF-κB signal pathway. In addition, RIP2 was involved in the regulation of the production of TNFα-2 and IL-1β1 in goldfish macrophages exposed to M. marinum.