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

Foot-and-mouth disease virus (FMDV) negatively regulates ZFP36 protein expression to alleviate its antiviral activity

Zinc finger protein 36 (ZFP36) is a key regulator of inflammatory and cytokine production. However, the interplay between swine zinc-finger protein 36 (sZFP36) and foot-and-mouth disease virus (FMDV) has not yet been reported. Here, we demonstrate that overexpression of sZFP36 restricted FMDV replication, while the knockdown of sZFP36 facilitated FMDV replication. To subvert the antagonism of sZFP36, FMDV decreased sZFP36 protein expression through its non-structural protein 3C protease (3Cpro). Our results also suggested that 3Cpro-mediated sZFP36 degradation was dependent on its protease activity. Further investigation revealed that both N-terminal and C-terminal-sZFP36 could be degraded by FMDV and FMDV 3Cpro. In addition, both N-terminal and C-terminal-sZFP36 decreased FMDV replication. Moreover, sZFP36 promotes the degradation of FMDV structural proteins VP3 and VP4 via the CCCH-type zinc finger and NES domains of sZFP36. Together, our results confirm that sZFP36 is a host restriction factor that negatively regulates FMDV replication.IMPORTANCEFoot-and-mouth disease (FMD) is an infectious disease of animals caused by the pathogen foot-and-mouth disease virus (FMDV). FMD is difficult to prevent and control because there is no cross-protection between its serotypes. Thus, we designed this study to investigate virus-host interactions. We first demonstrate that swine zinc-finger protein 36 (sZFP36) impaired FMDV structural proteins VP3 and VP4 to suppress viral replication. To subvert the antagonism of sZFP36, FMDV and FMDV 3Cpro downregulate sZFP36 expression to facilitate FMDV replication. Taken together, the present study reveals a previously unrecognized antiviral mechanism for ZFP36 and elucidates the role of FMDV in counteracting host antiviral activity.

Foot-and-mouth disease virus VP1 degrades YTHDF2 through autophagy to regulate IRF3 activity for viral replication

Many viruses, including foot-and-mouth disease virus (FMDV), can promote the degradation of host proteins through macroautophagy/autophagy, thereby promoting viral replication. However, the regulatory mechanism between autophagy and innate immune responses is not fully understood during FMDV infection. Here, we found that the host GTPBP4/NOG1 (GTP binding protein 4) is a negative regulator of innate immune responses. GTPBP4 deficiency promotes the antiviral innate immune response, resulting in the ability of GTPBP4 to promote FMDV replication. Meanwhile, GTPBP4-deficient mice are more resistant to FMDV infection. To antagonize the host's antiviral immunity, FMDV structural protein VP1 promotes the expression of GTPBP4, and the 209th site of VP1 is responsible for this effect. Mechanically, FMDV VP1 promotes autophagy during virus infection and interacts with and degrades YTHDF2 (YTH N6-methyladenosine RNA binding protein F2) in an AKT-MTOR-dependent autophagy pathway, resulting in an increase in GTPBP4 mRNA and protein levels. Increased GTPBP4 inhibits IRF3 binding to the Ifnb/Ifn-β promoter, suppressing FMDV-induced type I interferon production. In conclusion, our study revealed an underlying mechanism of how VP1 negatively regulates innate immunity through the autophagy pathway, which would contribute to understanding the negative regulation of host innate immune responses and the function of GTPBP4 and YTHDF2 during FMDV infection.Abbreviation: 3-MA:3-methyladenine; ACTB: actin beta; ATG: autophagy related; ChIP:chromatin immunoprecipitation; CQ: chloroquine; DAPI:4',6-diamidino-2-phenylindole; dpi: days post-infection; EV71:enterovirus 71; FMDV: foot-and-mouth disease virus; GTPBP4/NOG1: GTPbinding protein 4; HIF1A: hypoxia inducible factor 1 subunit alpha;hpt:hours post-transfection; IFNB/IFN-β:interferon beta; IRF3: interferon regulatory factor 3; MAP1LC3/LC3:microtubule associated protein 1 light chain 3; MAVS: mitochondriaantiviral signaling protein; MOI: multiplicity of infection; MTOR:mechanistic target of rapamycin kinase; m6A: N(6)-methyladenosine;qPCR:quantitativePCR; SIRT3:sirtuin 3; SQSTM1/p62: sequestosome 1; STING1: stimulator ofinterferon response cGAMP interactor 1; siRNA: small interfering RNA;TBK1: TANK binding kinase 1; TCID50:50% tissue culture infectious doses; ULK1: unc-51 like autophagyactivating kinase 1; UTR: untranslated region; WT: wild type; YTHDF2:YTH N6-methyladenosine RNA binding protein F2.

GRP78 recognizes EV-F 3D protein and activates NF-κB to repress virus replication by interacting with CHUK/IKBKB

Enteroviruses are the causative agents associated with several human and animal diseases, posing a significant threat to human and animal health. As one of the host immune defense strategies, innate immunity plays a crucial role in defending against invading pathogens, where the host utilizes a variety of mechanisms to inhibit or eliminate the pathogen. Here, we report a new strategy for the host to repress enterovirus replication by the 78 kDa glucose-regulated protein (GRP78), also known as heat shock protein family A member 5 (HSPA5). The GRP78 recognizes the EV-encoded RNA-dependent RNA polymerases (RdRPs) 3D protein and interacts with the nuclear factor kappa B kinase complex (CHUK) and subunit beta gene (IKBKB) to facilitate the phosphorylation and nuclear translocation of NF-κB, which induces the production of inflammatory factors and leads to a broad inhibition of enterovirus replication. These findings demonstrate a new role of GRP78 in regulating host innate immunity in response to viral infection and provide new insights into the mechanism underlying enterovirus replication and NF-κB activation.IMPORTANCEGRP78 is known as a molecular chaperone for protein folding and plays a critical role in maintaining protein folding and participating in cell proliferation, cell survival, apoptosis, and metabolism. However, the functions of GRP78 to participate in enterovirus genome replication and innate immune responses are rarely documented. In this study, we explored the functions of the EV-3D-interacting protein GRP78 and found that GRP78 inhibits enterovirus replication by activating NF-κB through binding to EV-F 3D and interacting with the NF-κB signaling molecules CHUK/IKBKB. This is the first report that GRP78 interacts with CHUK/IKBKB to activate the NF-κB signaling pathway, which leads to the expression of the proinflammatory cytokines and inhibition of enterovirus replication. These results demonstrate a unique mechanism of virus replication regulation by GRP78 and provide insights into the prevention and treatment of viral infections.

Picornavirus 2C proteins: structure-function relationships and interactions with host factors

Picornaviruses, which are positive-stranded, non-enveloped RNA viruses, are known to infect people and animals with a broad spectrum of diseases. Among the nonstructural proteins in picornaviruses, 2C proteins are highly conserved and exhibit multiple structural domains, including amphipathic α-helices, an ATPase structural domain, and a zinc finger structural domain. This review offers a comprehensive overview of the functional structures of picornaviruses' 2C protein. We summarize the mechanisms by which the 2C protein enhances viral replication. 2C protein interacts with various host factors to form the replication complex, ultimately promoting viral replication. We review the mechanisms through which picornaviruses' 2C proteins interact with the NF-κB, RIG-I, MDA5, NOD2, and IFN pathways, contributing to the evasion of the antiviral innate immune response. Additionally, we provide an overview of broad-spectrum antiviral drugs for treating various enterovirus infections, such as guanidine hydrochloride, fluoxetine, and dibucaine derivatives. These drugs may exert their inhibitory effects on viral infections by targeting interactions with 2C proteins. The review underscores the need for further research to elucidate the precise mechanisms of action of 2C proteins and to identify additional host factors for potential therapeutic intervention. Overall, this review contributes to a deeper understanding of picornaviruses and offers insights into the antiviral strategies against these significant viral pathogens.

Foot-and-mouth disease virus downregulates vacuolar protein sorting 28 to promote viral replication

Vacuolar protein sorting 28 (Vps28), a component of the ESCRT-I (endosomal sorting complex required for transport I), plays an important role in the pathogen life cycle. Here, we investigated the reciprocal regulation between Vps28 and the foot-and-mouth disease virus (FMDV). Overexpression of Vps28 decreased FMDV replication. On the contrary, the knockdown of Vps28 increased viral replication. Subsequently, the mechanistic study showed that Vps28 destabilized the replication complex (RC) by associating with 3A rather than 2C protein. In addition, Vps28 targeted FMDV VP0, VP1, and VP3 for degradation to inhibit viral replication. To counteract this, FMDV utilized tactics to restrict Vps28 to promote viral replication. FMDV degraded Vps28 mainly through the ubiquitin-proteasome pathway. Additional data demonstrated that 2B and 3A proteins recruited E3 ubiquitin ligase tripartite motif-containing protein 21 to degrade Vps28 at Lys58 and Lys25, respectively, and FMDV 3Cpro degraded Vps28 through autophagy and its protease activity. Meantime, the 3Cpro-mediated Vps28 degradation principally alleviated the ability to inhibit viral propagation. Intriguingly, we also demonstrated that the N-terminal and C-terminal domains of Vps28 were responsible for the suppression of FMDV replication, which suggested the elaborated counteraction between FMDV and Vps28. Collectively, our results first investigate the role of ESCRTs in host defense against picornavirus and unveil underlying strategies utilized by FMDV to evade degradation machinery for triumphant propagation. IMPORTANCE ESCRT machinery plays positive roles in virus entry, replication, and budding. However, little has been reported on its negative regulation effects during viral infection. Here, we uncovered the novel roles of ESCRT-I subunit Vps28 on FMDV replication. The data indicated that Vps28 destabilized the RC and impaired viral structural proteins VP0, VP1, and VP3 to inhibit viral replication. To counteract this, FMDV hijacked intracellular protein degradation pathways to downregulate Vps28 expression and thus promoted viral replication. Our findings provide insights into how ESCRT regulates pathogen life cycles and elucidate additional information regarding FMDV counteraction of host antiviral activity.

Foot-and-Mouth Disease Virus: Molecular Interplays with IFN Response and the Importance of the Model

Foot-and-mouth disease (FMD) is a highly contagious viral disease of cloven-hoofed animals with a significant socioeconomic impact. One of the issues related to this disease is the ability of its etiological agent, foot-and-mouth disease virus (FMDV), to persist in the organism of its hosts via underlying mechanisms that remain to be elucidated. The establishment of a virus–host equilibrium via protein–protein interactions could contribute to explaining these phenomena. FMDV has indeed developed numerous strategies to evade the immune response, especially the type I interferon response. Viral proteins target this innate antiviral response at different levels, ranging from blocking the detection of viral RNAs to inhibiting the expression of ISGs. The large diversity of impacts of these interactions must be considered in the light of the in vitro models that have been used to demonstrate them, some being sometimes far from biological systems. In this review, we have therefore listed the interactions between FMDV and the interferon response as exhaustively as possible, focusing on both their biological effect and the study models used.

Heat Shock Protein 60 Is Involved in Viral Replication Complex Formation and Facilitates Foot and Mouth Virus Replication by Stabilizing Viral Nonstructural Proteins 3A and 2C

The maintenance of viral protein homeostasis depends on the machinery of the infected host cells, giving us an insight into the interplay between host and virus. Accumulating evidence suggests that heat shock protein 60 (HSP60), as one molecular chaperone, is involved in regulating virus infection. However, the role of HSP60 during foot-and-mouth disease virus (FMDV) replication and its specific mechanisms have not been reported. We demonstrate that HSP60 modulates the FMDV life cycle. HSP60 plays a role at the postentry stage of the viral life cycle, including RNA replication and mRNA translation; however, HSP60 does not affect viral replication of Seneca Valley virus (SVA) or encephalomyocarditis virus (EMCV). We found that HSP60 is involved in FMDV replication complex (RC) formation. Furthermore, our results indicate that HSP60 interacts with FMDV nonstructural proteins 3A and 2C, key elements of the viral replication complex. We also show that HSP60 regulates the stability of 3A and 2C via caspase-dependent and autophagy-lysosome-dependent degradation, thereby promoting FMDV RNA synthesis and mRNA translation mediated by the RC. Additionally, we determined that the apical domain of HSP60 is responsible for interacting with 3A and 2C. The N terminus of 3A and ATPase domain of 2C are involved in binding to HSP60. Importantly, HSP60 depletion potently reduced FMDV pathogenicity in infected mice. Altogether, this study demonstrates a specific role of HSP60 in promoting FMDV replication. Furthermore, targeting host HSP60 will help us design the FMDV-specific antiviral drugs.

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.

An anti-picornaviral strategy based on the crystal structure of foot-and-mouth disease virus 2C protein

The foot-and-mouth disease virus (FMDV) 2C protein shares conserved motifs with enterovirus 2Cs despite low sequence identity. Here, we determine the crystal structure of an FMDV 2C fragment to 1.83 Å resolution, which comprises an ATPase domain, a region equivalent to the enterovirus 2C zinc-finger (ZFER), and a C-terminal domain harboring a loop (PBL) that occupies a hydrophobic cleft (Pocket) in an adjacent 2C molecule. Mutations at ZFER, PBL, and Pocket affect FMDV 2C ATPase activity and are lethal to FMDV infectious clones. Because the PBL-Pocket interaction between FMDV 2C molecules is essential for its functions, we design an anti-FMDV peptide derived from PBL (PBL-peptide). PBL-peptide inhibits FMDV 2C ATPase activity, binds FMDV 2C with nanomolar affinity, and disrupts FMDV 2C oligomerization. FMDV 2C targets lipid droplets (LDs) and induces LD clustering in cells, and PBL-peptide disrupts FMDV 2C-induced LD clustering. Finally, we demonstrate that PBL-peptide exhibits anti-FMDV activity in cells.

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.

The Insufficient Activation of RIG-I-Like Signaling Pathway Contributes to Highly Efficient Replication of Porcine Picornaviruses in IBRS-2 Cells

Seneca Valley virus (SVV) or commonly known as senecavirus A, is one of the picornavirus that is associated with vesicular disease and neonatal mortality in swine herds. Our previous study found that SVV replicates extremely faster in porcine Instituto Biologico-Rim Suino-2 (IBRS-2) cells than that in porcine kidney-15 (PK-15) cells. However, the underlying mechanism remains unknown. In this study, we comprehensively compared the expression features between IBRS-2 cells and PK-15 cells in response to SVV infection by an unbiased high-throughput quantitative proteomic analysis. We found that the innate immune response–related pathways were efficiently activated in PK-15 cells but not in IBRS-2 cells during SVV infection. A large amount of interferon (IFN)-stimulated genes were induced in PK-15 cells. In contrast, no IFN-stimulated genes were induced in IBRS-2 cells. Besides, we determined similar results in the two cell lines infected by another porcine picornavirus foot-and-mouth disease virus. Further study demonstrated that the Janus kinase signal transducer and activator of transcription signaling pathway was functioning properly in both IBRS-2 and PK-15 cells. A systematic screening study revealed that the aberrant signal transduction from TANK-binding kinase 1 to IFN regulatory factor 3 in the retinoic acid–inducible gene I–like receptor signaling pathway in IBRS-2 cells was the fundamental cause of the different innate immune response manifestation and different viral replication rate in the two cell lines. Together, our findings determined the different features of IBRS-2 and PK-15 cell lines, which will help for clarification of the pathogenesis of SVV. Besides, identification of the underlying mechanisms will provide new targets and an insight for decreasing the viral clearance rate and probably improve the oncolytic effect by SVV in cancer cells.

•

Divergent innate immune responses were triggered by SVV in IBRS-2 and PK-15 cells.

•

SVV induced higher levels of type I IFN in PK-15 cells than in IBRS-2 cells.

•

IBRS-2 cell line has an aberrant RLR pathway but an intact type I IFN pathway.

•

TBK1-mediated antiviral signal transduction was dysfunctional in IBRS-2 cells.

DDX21, a Host Restriction Factor of FMDV IRES-Dependent Translation and Replication

In cells, the contributions of DEAD-box helicases (DDXs), without which cellular life is impossible, are of utmost importance. The extremely diverse roles of the nucleolar helicase DDX21, ranging from fundamental cellular processes such as cell growth, ribosome biogenesis, protein translation, protein–protein interaction, mediating and sensing transcription, and gene regulation to viral manipulation, drew our attention. We designed this project to study virus–host interactions and viral pathogenesis. A pulldown assay was used to investigate the association between foot-and-mouth disease virus (FMDV) and DDX21. Further insight into the DDX21–FMDV interaction was obtained through dual-luciferase, knockdown, overexpression, qPCR, and confocal microscopy assays. Our results highlight the antagonistic feature of DDX21 against FMDV, as it progressively inhibited FMDV internal ribosome entry site (IRES) -dependent translation through association with FMDV IRES domains 2, 3, and 4. To subvert this host helicase antagonism, FMDV degraded DDX21 through its non-structural proteins 2B, 2C, and 3C protease (3C^pro). Our results suggest that DDX21 is degraded during 2B and 2C overexpression and FMDV infection through the caspase pathway; however, DDX21 is degraded through the lysosomal pathway during 3C^pro overexpression. Further investigation showed that DDX21 enhanced interferon-beta and interleukin-8 production to restrict viral replication. Together, our results demonstrate that DDX21 is a novel FMDV IRES trans-acting factor, which negatively regulates FMDV IRES-dependent translation and replication.

Ribosomal Protein L13 Participates in Innate Immune Response Induced by Foot-and-Mouth Disease Virus

In addition to ribosomal protein synthesis and protein translation, ribosomal proteins also participate in tumorigenesis and tumor progression, immune responses, and viral replication. Here, we show that ribosomal protein L13 (RPL13) participates in the antiviral immune response induced by foot-and-mouth disease virus (FMDV), inhibiting FMDV replication. The overexpression of RPL13 promoted the induction and activation of the promoters of the nuclear factor-κB (NF-κB) and interferon-β (IFN-β) genes, and the expression and protein secretion of the antiviral factor IFN-β and proinflammatory cytokine interleukin-6 (IL-6). The knockdown of RPL13 had the opposite effects. We also found that the FMDV 3C^pro protease interacts with RPL13, and that its activity reduces the expression of RPL13, thus antagonizing the RPL13-mediated antiviral activity. This study extends our knowledge of the extraribosomal functions of ribosomal proteins and provides new scientific information on cellular antiviral defenses and virus-antagonizing mechanisms.

Identification of novel interactions between host and non-structural protein 2C of foot-and-mouth disease virus

The 2C protein of foot-and-mouth disease virus (FMDV) is reported to play a critical role in the virus replication complex and modulating the host's immune response. However, the underlying molecular intricacies of subversion of cellular machinery remains poorly understood, thus emphasizing the need to study 2C-host interactions. In this study, we identified the host proteins interacting with the 2C using yeast-two hybrid (Y2H) approach, which is one of the most recognized, high-throughput tools to study protein-protein interactions. The FMDV-2C bait was characterized for auto-activation, toxicity, and expression and was found to be suitable for mating with cDNA library. On preliminary screening a total of 32 interacting host proteins were identified which were reduced to 22 on subsequent confirmation with alternative yeast based assays. Amongst these, NMI/2C interaction has been reported earlier by Wang et al. (2012) and remaining 21 are novel interactions. The Reactome analysis has revealed the role of the identified host proteins in cellular pathways exploited by 2C during FMDV replication. We also confirmed interaction of MARCH7, an E3 ubiquitin ligase with 2C using mammalian two-hybrid system and co-immunoprecipitation. This study leads to the identification of novel 2C interacting host proteins which enhance our understanding of 2C-host interface and may provide checkpoints for development of potential therapeutics against FMDV.

Picornavirus 3C - a protease ensuring virus replication and subverting host responses

The protease 3C is encoded by all known picornaviruses, and the structural features related to its protease and RNA-binding activities are conserved; these contribute to the cleavage of viral polyproteins and the assembly of the viral RNA replication complex during virus replication. Furthermore, 3C performs functions in the host cell through its interaction with host proteins. For instance, 3C has been shown to selectively 'hijack' host factors involved in gene expression, promoting picornavirus replication, and to inactivate key factors in innate immunity signaling pathways, inhibiting the production of interferon and inflammatory cytokines. Importantly, 3C maintains virus infection by subtly subverting host cell death and modifying critical molecules in host organelles. This Review focuses on the molecular mechanisms through which 3C mediates physiological processes involved in virus-host interaction, thus highlighting the picornavirus-mediated pathogenesis caused by 3C.

Virus-Host Interactions in Foot-and-Mouth Disease Virus Infection

Foot-and-mouth disease (FMD) is a highly contagious disease of cloven-hoofed animals, which has been regarded as a persistent challenge for the livestock industry in many countries. Foot-and-mouth disease virus (FMDV) is the etiological agent of FMD that can spread rapidly by direct and indirect transmission. FMDV is internalized into host cell by the interaction between FMDV capsid proteins and cellular receptors. When the virus invades into the cells, the host antiviral system is quickly activated to suppress the replication of the virus and remove the virus. To retain fitness and host adaptation, various viruses have evolved multiple elegant strategies to manipulate host machine and circumvent the host antiviral responses. Therefore, identification of virus-host interactions is critical for understanding the host defense against virus infections and the pathogenesis of the viral infectious diseases. This review elaborates on the virus-host interactions during FMDV infection to summarize the pathogenic mechanisms of FMD, and we hope it can provide insights for designing effective vaccines or drugs to prevent and control the spread of FMD and other diseases caused by picornaviruses.

Role of NLRs in the Regulation of Type I Interferon Signaling, Host Defense and Tolerance to Inflammation

Type I interferon signaling contributes to the development of innate and adaptive immune responses to either viruses, fungi, or bacteria. However, amplitude and timing of the interferon response is of utmost importance for preventing an underwhelming outcome, or tissue damage. While several pathogens evolved strategies for disturbing the quality of interferon signaling, there is growing evidence that this pathway can be regulated by several members of the Nod-like receptor (NLR) family, although the precise mechanism for most of these remains elusive. NLRs consist of a family of about 20 proteins in mammals, which are capable of sensing microbial products as well as endogenous signals related to tissue injury. Here we provide an overview of our current understanding of the function of those NLRs in type I interferon responses with a focus on viral infections. We discuss how NLR-mediated type I interferon regulation can influence the development of auto-immunity and the immune response to infection.

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.

The Structure, Function, and Mechanisms of Action of Enterovirus Non-structural Protein 2C

Enteroviruses are a group of RNA viruses belonging to the family Picornaviridae. They include human enterovirus groups A, B, C, and D as well as non-human enteroviruses. Enterovirus infections can lead to hand, foot, and mouth disease and herpangina, whose clinical manifestations are often mild, although some strains can result in severe neurological complications such as encephalitis, myocarditis, meningitis, and poliomyelitis. To date, research on enterovirus non-structural proteins has mainly focused on the 2A and 3C proteases and 3D polymerase. However, another non-structural protein, 2C, is the most highly conserved protein, and plays a vital role in the enterovirus life cycle. There are relatively few studies on this protein. Previous studies have demonstrated that enterovirus 2C is involved in virus uncoating, host cell membrane rearrangements, RNA replication, encapsidation, morphogenesis, ATPase, helicase, and chaperoning activities. Despite ongoing research, little is known about the pathogenesis of enterovirus 2C proteins in viral replication or in the host innate immune system. In this review, we discuss and summarize the current understanding of the structure, function, and mechanism of the enterovirus 2C proteins, focusing on the key mutations and motifs involved in viral infection, replication, and immune regulation. We also focus on recent progress in research into the role of 2C proteins in regulating the pattern recognition receptors and type I interferon signaling pathway to facilitate viral replication. Given these functions and mechanisms, the potential application of the 2C proteins as a target for anti-viral drug development is also discussed. Future studies will focus on the determination of more crystal structures of enterovirus 2C proteins, which might provide more potential targets for anti-viral drug development against enterovirus infections.

Tomatidine inhibits porcine epidemic diarrhea virus replication by targeting 3CL protease

Porcine epidemic diarrhea virus (PEDV) causes lethal diarrhea in suckling piglets, leading to severe economic losses worldwide. There is an urgent need to find new therapeutic methods to prevent and control PEDV. Not only is there a shortage of commercial anti-PEDV drugs, but available commercial vaccines fail to protect against highly virulent PEDV variants. We screened an FDA-approved library of 911 natural products and found that tomatidine, a steroidal alkaloid extracted from the skin and leaves of tomatoes, demonstrates significant inhibition of PEDV replication in Vero and IPEC-J2 cells in vitro. Molecular docking and molecular dynamics analysis predicted interactions between tomatidine and the active pocket of PEDV 3CL protease, which were confirmed by fluorescence spectroscopy and isothermal titration calorimetry (ITC). The inhibiting effect of tomatidine on 3CL protease was determined using cleavage visualization and FRET assay. Tomatidine-mediated blocking of 3CL protease activity in PEDV-infected cells was examined by western blot detection of the viral polyprotein in PEDV-infected cells. It indicates that tomatidine inhibits PEDV replication mainly by targeting 3CL protease. In addition, tomatidine also has antiviral activity against transmissible gastroenteritis virus (TGEV), porcine reproductive and respiratory syndrome virus (PRRSV), encephalo myocarditis virus (EMCV) and seneca virus A (SVA) in vitro. These results may be helpful in developing a new prophylactic and therapeutic strategy against PEDV and other swine disease infections.

Advances in Foot-and-Mouth Disease Virus Proteins Regulating Host Innate Immunity

Foot-and-mouth disease (FMD) is a highly contagious disease that affects cloven-hoofed animals such as pigs, cattle, and sheep. The disease is caused by the foot-and-mouth disease virus (FMDV) which has a non-enveloped virion with icosahedral symmetry that encapsulates a positive-sense, single-stranded RNA genome of ∼8.4 kb. FMDV infection causes obvious immunosuppressive effects on the host. In recent years, studies on the immunosuppressive mechanism of FMDV have become a popular topic. In addition, studies have shown that many FMDV proteins are involved in the regulation of host innate immunity and have revealed mechanisms by which FMDV proteins mediate host innate immunity. In this review, advances in studies on the mechanisms of interaction between FMDV proteins and host innate immunity are summarized to provide a comprehensive understanding of FMDV pathogenesis and the theoretical basis for FMD prevention and control.

Molecular Mechanisms of Immune Escape for Foot-and-Mouth Disease Virus

Foot-and-mouth disease virus (FMDV) causes a highly contagious vesicular disease in cloven-hoofed livestock that results in severe consequences for international trade, posing a great economic threat to agriculture. The FMDV infection antagonizes the host immune responses via different signaling pathways to achieve immune escape. Strategies to escape the cell immune system are key to effective infection and pathogenesis. This review is focused on summarizing the recent advances to understand how the proteins encoded by FMDV antagonize the host innate and adaptive immune responses.

Use of IFN-Based Biotherapeutics to Harness the Host Against Foot-And-Mouth Disease

Foot-and-mouth disease (FMD) is a highly contagious vesicular disease of cloven-hoofed animals that severely constrains international trade of livestock and animal products. Currently, disease control measures include broad surveillance, enforcement of sanitary policy, and use of an inactivated vaccine. While use of these measures has contributed to eliminating foot-and-mouth disease virus (FMDV) from a vast area of the world, the disease remains endemic in three continents, and outbreaks occasionally appear in previously declared FMD-free zones, causing economic and social devastation. Among others, a very fast rate of viral replication and the need for 7 days to achieve vaccine-induced protection are the main limitations in controlling the disease. New fast-acting antiviral strategies targeted to boost the innate immunity of the host to block viral replication are needed. Here we review the knowledge on the multiple strategies FMDV has evolved to block the host innate immunity, with particularly focus on the past and current research toward the development of interferon (IFN)-based biotherapeutics in relevant livestock species.

NOD1 and NOD2 Activation by Diverse Stimuli: a Possible Role for Sensing Pathogen-Induced Endoplasmic Reticulum Stress

Prompt recognition of microbes by cells is critical to eliminate invading pathogens. Some cell-associated pattern recognition receptors (PRRs) recognize and respond to microbial ligands. However, others can respond to cellular perturbations, such as damage-associated molecular patterns (DAMPs). Nucleotide oligomerization domains 1 and 2 (NOD1/2) are PRRs that recognize and respond to multiple stimuli of microbial and cellular origin, such as bacterial peptidoglycan, viral infections, parasitic infections, activated Rho GTPases, and endoplasmic reticulum (ER) stress. How NOD1/2 are stimulated by such diverse stimuli is not fully understood but may partly rely on cellular changes during infection that result in ER stress. NOD1/2 are ER stress sensors that facilitate proinflammatory responses for pathogen clearance; thus, NOD1/2 may help mount broad antimicrobial responses through detection of ER stress, which is often induced during a variety of infections. Some pathogens may subvert this response to promote infection through manipulation of NOD1/2 responses to ER stress that lead to apoptosis. Here, we review NOD1/2 stimuli and cellular responses. Furthermore, we discuss pathogen-induced ER stress and how it might potentiate NOD1/2 signaling.

NLRP3 inflammasome activation by Foot-and-mouth disease virus infection mainly induced by viral RNA and non-structural protein 2B

Foot-and-mouth disease virus (FMDV) is a positive-strand RNA virus of the family Picornaviridae. Early studies show that some viruses of Picornaviridae, such as EMCV and EV71, induce NLRP3 inflammasome activation. Our current study demonstrates that FMDV induces the secretion of caspase-1 and interleukin 1 beta (IL-1β), as well as activates the NLRP3 inflammasome in a dose- and time-dependent manner. Meanwhile, NLRP3 inflammasome can suppress FMDV replication during virus infection. Both FMDV RNA and viroporin 2B stimulate NLRP3 inflammasome activation. FMDV RNA triggers NLRP3 inflammasome through p-NF-κB/p65 pathway not dependent on RIG-I inflammasome. FMDV 2B activates NLRP3 inflammasome through elevation of intracellular ion, but not dependent on mitochondrial reactive oxygen species (ROS) and lysosomal cathepsin B. It further demonstrates that 2B viroporin activates NLRP3 inflammasome and induces IL-1β in mice, which enhances the specific immune response against FMDV as an ideal self-adjuvant for FMD VLPs vaccine in guinea pigs. The results reveal a series of regulations between NLRP3 inflammasome complex and FMDV. Amino acids 140-145 of 2B is essential for forming an ion channel. By mutating the amino acid and changing the hydrophobic properties, the helical transmembrane region of the viroporin 2B is altered, so that the 2B is insufficient to trigger the activation of NLRP3 inflammasome. This study demonstrates the functions of FMDV RNA and 2B viroporin activate NLRP3 inflammasome and provides some useful information for the development of FMD vaccine self-adjuvant, which is also helpful for the establishment of effective prevention strategies by targeting NLRP3 inflammasome.