The tyrosine kinase SRC of grass carp (Ctenopharyngodon idellus) up-regulates the expression of IFN I by activating TANK binding kinase 1

Host factor MxA restricts Dabie bandavirus infection by targeting the viral NP protein to inhibit NP-RdRp interaction and ribonucleoprotein activity

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease with high case mortality rates, which is caused by Dabie bandavirus (DBV), a novel pathogen also termed as SFTS virus (SFTSV). Currently, no specific therapeutic drugs or vaccines are available for SFTS. Myxovirus resistance protein A (MxA) has been shown to inhibit multiple viral pathogens; however, the role of MxA in DBV infection is unknown. Here, we demonstrated that DBV stimulates MxA expression which, in turn, restricts DBV infection. Mechanistic target analysis revealed that MxA specifically interacts with the viral nucleocapsid protein (NP) in a manner independent of RNA. Minigenome reporter assay showed that in agreement with its targeting of NP, MxA inhibits DBV ribonucleoprotein (RNP) activity. In detail, MxA interacts with the NP N-terminal and disrupts the interaction of NP with the viral RNA-dependent RNA polymerase (RdRp) but not NP multimerization, the critical activities of NP for RNP formation and function. Furthermore, MxA N-terminal domain was identified as the functional domain inhibiting DBV infection, and, consistently, then was shown to interact with NP and obstruct the NP-RdRp interaction. Additionally, threonine 103 within the N-terminal domain is important for MxA inhibition to DBV, and its mutation (T103A) attenuates MxA binding to NP and obstruction of the NP-RdRp interaction. This study uncovers MxA inhibition of DBV with a series of functional and mechanistical analyses, providing insights into the virus-host interactions and probably helping inform the development of antiviral agents in the future.IMPORTANCEDBV/SFTSV is an emerging high-pathogenic virus. Since its first identification in China in 2009, cases of DBV infection have been reported in many other countries, posing a significant threat to public health. Uncovering the mechanisms of DBV-host interactions is necessary to understand the viral pathogenesis and host response and may advance the development of antiviral therapeutics. Here, we found that host factor MxA whose expression is induced by DBV restricts the virus infection. Mechanistically, MxA specifically interacts with the viral NP and blocks the NP-RdRp interaction, inhibiting the viral RNP activity. Further studies identified the key domain and amino acid residue required for MxA inhibition to DBV. Consistently, they were then shown to be important for MxA targeting of NP and obstruction of the NP-RdRp association. These findings unravel the restrictive role of MxA in DBV infection and the underlying mechanism, expanding our knowledge of the virus-host interactions.

Post-translational modifications and regulations of RLR signaling molecules in cytokines-mediated response in fish

Teleosts rely on innate immunity to recognize and defense against pathogenic microorganisms. RIG-I-like receptor (RLR) family is the major pattern recognition receptor (PRR) to detect RNA viruses. After recognition of viral RNA components, these cytosolic sensors activate downstream signaling cascades to induce the expression of type I interferons (IFNs) and other cytokines firing antiviral responses. Meanwhile, numerous molecules take part in the complex regulation of RLR signals by various methods, such as post-translational modification (PTM), to produce an immune response that is appropriately balanced. In this review, we summarize our recent understanding of PTMs and other regulatory proteins in modulating RLR signaling pathway, which is helpful for systematically studying the regulatory mechanism of antiviral innate immunity of teleost fish.

Grass carp PRMT6 negatively regulates innate immunity by inhibiting the TBK1/IRF3 binding and cutting down IRF3 phosphorylation level

Subcellular localization analysis implicated that CiPRMT6 was mainly located in the nucleus, with a small part of them located in the cytoplasm. PRMT6, namely protein arginine methyltransferase 6, was first identified and demonstrated to catalyze the methylation of arginine residue on the chromatin histones in mammals. Mammalian PRMT6 usually acts as an arginine methyltransferase in the nucleus, but induces antiviral innate immune response in the cytoplasm. Nowadays, there have been few reports about PRMT6 in teleost. In this study, we investigated the potential molecular mechanisms underlying the interaction of PRMT6 expression and IFN1 response in grass carp. We first cloned and identified a grass carp PRMT6 (named CiPRMT6, MN781672.1), which is 1068bp in length encoding a deduced polypeptide of 355 amino acids. In CIK cell, CiPRMT6 expression was up-regulated upon stimulation with poly (I:C); while overexpression of PRMT6 suppressed the promoter activity of grass carp IFN1 and reduced the phosphorylation of IRF3; however, the amount of PRMT6 mutant (lack of methyltransferase domain) was increased in the cytoplasm. Our results also showed that grass carp PRMT6 and IRF3 (but not TBK1) were co-located and bound to each other in the cytoplasm. The binding of CiPRMT6 to IRF3 impairs the interaction between TBK1 and IRF3, indicating that CiPRMT6 is a negative regulator for IFN1 expression through TBK1-IRF3 signaling pathway in grass carp. In conclusion, we identified that CiPRMT6 negatively regulated IFN1 expression by inhibiting the TBK1-IRF3 interaction as well as IRF3 phosphorylation.

Fish Paralog Proteins RNASEK-a and -b Enhance Type I Interferon Secretion and Promote Apoptosis

Type I interferon and apoptosis elicit multifaceted effects on host defense and various diseases, such as viral infections and cancers. However, the gene/protein network regulating type I interferon and apoptosis has not been elucidated completely. In this study, we selected grass carp (Ctenopharyngodon idella) as an experimental model to investigate the modulation of RNASEK on the secretion of type I interferon and apoptosis. We first cloned two paralogs RNASEK-a and -b in grass carp, defined three exons in each gene, and found the length of both coding regions is 306 bp with 73.27% of protein homology. The protein sequences of the two paralogs are highly conserved across species. Two proteins were mainly localized in early and late endosomes and endoplasmic reticulum. Further, quantitative real-time PCR demonstrated that dsRNA poly I:C and grass carp reovirus upregulated RNASEK-a and -b in grass carp cells and tissues. Overexpression of RNASEK-a and -b individually induced type I interferon expression and the phosphorylation of IRF3/IRF7 shown by Western blot and immunofluorescent staining, increased Bax/Bcl-2 mRNA ratio, DNA fragmentations, TUNEL-positive cells, and the proportion of Annexin V-positive signals in flow cytometry, and activated eIF2α, opposite to that observed when RNASEK-a and -b were knocked down in multiple cell types. Taken together, we claim for the first time that fish paralog proteins RNASEK-a and -b enhance type I interferon secretion and promote apoptosis, which may be involved in the phosphorylation of IRF3/IRF7 and eIF2α, respectively. Our study reveals a previously unrecognized role of RNASEK as a new positive regulator of type I interferon and apoptosis.

Grass carp (Ctenopharyngodon idellus) SHP2 suppresses IFN I expression via decreasing the phosphorylation of GSK3β in a non-contact manner

As a tyrosine phosphatase, Src homology 2-containing protein tyrosine phosphatase 2 (SHP2) serves as an inhibitor in PI3K-Akt pathway. In mammals, SHP2 can phosphorylate GSK3β at Y216 site to control the expression of IFN. So far, the multiple functions of SHP2 have been reported in mammals. However, little is known about fish SHP2. In this study, we cloned and identified a grass carp (Ctenopharyngodon idellus) SHP2 gene (CiSHP2, MT373151). SHP2 is conserved among different vertebrates by amino acid sequences alignment and the phylogenetic tree analysis. CiSHP2 shared the closest homology with Danio rerio SHP2. Simultaneously, SHP2 was also tested in grass carp tissues and CIK (C. idellus kidney) cells. We found that it responded to poly I:C stimulation. CiSHP2 was located in the cytoplasm just as the same as those of mammals. Interestingly, it inhibited the phosphorylation level of GSK3β in a non-contact manner. Meanwhile CiGSK3β interacted with and directly phosphorylated CiTBK1. In addition, we found that CiSHP2 also reduced the phosphorylation level of CiTBK1 by CiGSK3β, and then it depressed the expression of IFN I via GSK3β-TBK1 axis. These results suggested that CiSHP2 was involved in CiGSK3β and CiTBK1 activity but not regulated their transcriptional level. At the same time, we also found that CiSHP2 also influenced the activity of CiIRF3. Therefore, fish SHP2 inhibited IFN I expression through blocking GSK3β-TBK1 signal axis.