Ctenopharyngodon idella PKZ facilitates cell apoptosis through phosphorylating eIF2α

Protein kinase double-stranded RNA-dependent (PKR) in antiviral defence in fish and mammals

The interferon-inducible double-stranded RNA-dependent protein kinase (PKR) is one of the key antiviral arms of the innate immune system. Upon binding of viral double stranded RNA, a viral Pattern Associated Molecular Pattern (PAMP), PKR gets activated and phosphorylates the eukaryotic initiation factor 2α (eIF2α) resulting in a protein shut-down that limits viral replication. Since its discovery in the mid-seventies, PKR has been shown to be involved in multiple important cellular processes including apoptosis, proinflammatory and innate immune responses. Viral subversion mechanisms of PKR underline its importance in the antiviral response of the host. PKR activation pathways and its mechanisms of action were previously identified and characterised mostly in mammalian models. However, fish Pkr and fish-specific paralogue Z-DNA-dependent protein kinase (Pkz) also play key role in antiviral defence. This review gives an update on the current knowledge on fish Pkr/Pkz, their conditions of activation and their implication in the immune responses to viruses, in comparison to their mammalian counterparts.

Susceptibility and Permissivity of Zebrafish (Danio rerio) Larvae to Cypriniviruses

The zebrafish (Danio rerio) represents an increasingly important model organism in virology. We evaluated its utility in the study of economically important viruses from the genus Cyprinivirus (anguillid herpesvirus 1, cyprinid herpesvirus 2 and cyprinid herpesvirus 3 (CyHV-3)). This revealed that zebrafish larvae were not susceptible to these viruses after immersion in contaminated water, but that infections could be established using artificial infection models in vitro (zebrafish cell lines) and in vivo (microinjection of larvae). However, infections were transient, with rapid viral clearance associated with apoptosis-like death of infected cells. Transcriptomic analysis of CyHV-3-infected larvae revealed upregulation of interferon-stimulated genes, in particular those encoding nucleic acid sensors, mediators of programmed cell death and related genes. It was notable that uncharacterized non-coding RNA genes and retrotransposons were also among those most upregulated. CRISPR/Cas9 knockout of the zebrafish gene encoding protein kinase R (PKR) and a related gene encoding a protein kinase containing Z-DNA binding domains (PKZ) had no impact on CyHV-3 clearance in larvae. Our study strongly supports the importance of innate immunity-virus interactions in the adaptation of cypriniviruses to their natural hosts. It also highlights the potential of the CyHV-3-zebrafish model, versus the CyHV-3-carp model, for study of these interactions.

Construction of ssDNA-Attached LR-Chimera Involving Z-DNA for ZBP1 Binding Analysis

The binding of proteins to Z-DNA is hard to analyze, especially for short non-modified DNA, because it is easily transferred to B-DNA. Here, by the hybridization of a larger circular single-stranded DNA (ssDNA) with a smaller one, an LR-chimera (involving a left-handed part and a right-handed one) with an ssDNA loop is produced. The circular ssDNAs are prepared by the hybridization of two ssDNA fragments to form two nicks, followed by nick sealing with T4 DNA ligase. No splint (a scaffold DNA for circularizing ssDNA) is required, and no polymeric byproducts are produced. The ssDNA loop on the LR-chimera can be used to attach it with other molecules by hybridization with another ssDNA. The gel shift binding assay with Z-DNA specific binding antibody (Z22) or Z-DNA binding protein 1 (ZBP1) shows that stable Z-DNA can form under physiological ionic conditions even when the extra ssDNA part is present. Concretely, a 5'-terminal biotin-modified DNA oligonucleotide complementary to the ssDNA loop on the LR-chimera is used to attach it on the surface of a biosensor inlaid with streptavidin molecules, and the binding constant of ZBP1 with Z-DNA is analyzed by BLI (bio-layer interferometry). This approach is convenient for quantitatively analyzing the binding dynamics of Z-DNA with other molecules.

Grass Carp (Ctenopharyngodon idella) KAT8 Inhibits IFN 1 Response Through Acetylating IRF3/IRF7

Post-translational modifications (PTMs), such as phosphorylation and ubiquitination, etc., have been reported to modulate the activities of IRF3 and IRF7. In this study, we found an acetyltransferase KAT8 in grass carp (CiKAT8, MW286472) that acetylated IRF3/IRF7 and then resulted in inhibition of IFN 1 response. CiKAT8 expression was up-regulated in the cells under poly I:C, B-DNA or Z-DNA stimulation as well as GCRV(strain 873) or SVCV infection. The acetyltransferase domain (MYST domain) of KAT8 promoted the acetylation of IRF3 and IRF7 through the direct interaction with them. So, the domain is essential for KAT8 function. Expectedly, KAT8 without MYST domain (KAT8-△264-487) was granularly aggregated in the nucleus and failed to down-regulate IFN 1 expression. Subcellular localization analysis showed that KAT8 protein was evenly distributed in the nucleus. In addition, we found that KAT8 inhibited the recruitment of IRF3 and IRF7 to ISRE response element. Taken together, our findings revealed that grass carp KAT8 blocked the activities of IRF3 and IRF7 by acetylating them, resulting in a low affinity interaction of ISRE response element with IRF3 and IRF7, and then inhibiting nucleic acids-induced innate immune response.

Cloning and functional analysis of SNX8 from grass carp (Ctenopharyngodon idellus)

Sorting nexin 8 (SNX8), a member of sorting nexin protein family, plays important roles in endocytosis, endosomal sorting, and innate immune response. To date, a few homologs of SNX8 have been found in fish except in mammals. In this study, a teleost SNX8 cDNA was identified from grass carp (Ctenopharyngodon idellus). CiSNX8 was up-regulated significantly after infection with poly I:C or GCRV. We found that SNX8 was mainly distributed in the endoplasmic reticulum (ER) in CIK cells. Further analysis indicated that CiSNX8 might negatively regulate RLR signaling pathway that is quite distinct from mammalian SNX8. In addition, CiSNX8 could interact with MAVS, STING, TBK1, IRF3 and IRF7. Either wild type CiSNX8 or mutants of N-terminal PX domain (aa 1-245) and C-terminal BAR domain (aa 256-519) could associate with STING. These results suggested that fish SNX8 participated in innate immune response through different molecular mechanisms.

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.

cGASa and cGASb from grass carp (Ctenopharyngodon idellus) play opposite roles in mediating type I interferon response

Cyclic GMP-AMP synthase (cGAS) is known as a DNA sensor for the initiation of innate immune responses in human and other mammals. However, the knowledge about fish cGAS is limited. In this study, we identified two paralogs of cGAS genes from grass carp (Ctenopharyngodon idellus), namely, CicGASa and CicGASb. Grass carp cGASa and cGASb share some conservative domains with mammalian cGASs; however, cGASb contains a unique transmembrane domain. Grass carp cGASa and cGASb responded to GCRV and poly (dA:dT) infection, but they played opposite roles in the regulation of type I IFN response, i.e. cGASa served as an activator for ISGs and NF-κB in a dose-dependent manner, while cGASb acted as an inhibitor. We found that cGASa and cGASb interacted with STING. Similarly, cGASa is an activator for IRF7, but cGASb inhibited IRF7 expression. Both cGASa and STING can protect cells from GCRV infection. Grass carp cGASb inhibited cGASa-induced type I IFN response by the competitive interaction with STING, suggesting that cGASb may be a negative regulator of cGASa-STING-IRF7 axis.

Ctenopharyngodon idella Tollip regulates MyD88-induced NF-κB activation

Toll-interacting protein (Tollip) and MyD88 are key components of the TLR/IL-1R signaling pathway in mammals. MyD88 is known as a universal adaptor protein involving in TLR/IL-1R-induced NF-κB activation. Tollip is a crucial negative regulator of TLR-mediated innate immune responses. Previous studies have demonstrated that teleost Tollip served as a negative regulator of MyD88-dependent TLR signaling pathway. However, the mechanism is still unclear. In particular, the effect of TBD, C2, and CUE domains of Tollip on MyD88-NF-κB signaling pathway remains to be elucidated. In this study, we found that the response of grass carp Tollip (CiTollip) to LPS stimulation was faster and stronger than that of poly I:C treatment, and CiTollip diminished the expression of tnf-α induced by LPS. Further assays indicated that except for the truncated mutant of △CUE2 (1-173 aa), wild type CiTollip and other truncated mutants (△N-(52-276 aa), △C2-(173-276 aa) and △CUE1-(1-231 aa)) could associate with MyD88 and negatively regulate MyD88-induced NF-κB activation. It suggested that the C-terminal (173-276 aa), in particular the connection section between C2 and CUE domains (173-231 aa), played a pivotal role in suppressing MyD88-induced activation of NF-κB.

The Role of the Z-DNA Binding Domain in Innate Immunity and Stress Granules

Both DNA and RNA can maintain left-handed double helical Z-conformation under physiological condition, but only when stabilized by Z-DNA binding domain (ZDBD). After initial discovery in RNA editing enzyme ADAR1, ZDBD has also been described in pathogen-sensing proteins ZBP1 and PKZ in host, as well as virulence proteins E3L and ORF112 in viruses. The host-virus antagonism immediately highlights the importance of ZDBD in antiviral innate immunity. Furthermore, Z-RNA binding has been shown to be responsible for the localization of these ZDBD-containing proteins to cytoplasmic stress granules that play central role in coordinating cellular response to stresses. This review sought to consolidate current understanding of Z-RNA sensing in innate immunity and implore possible roles of Z-RNA binding within cytoplasmic stress granules.

Grass carp (Ctenopharyngodon idella) GPATCH3 initiates IFN 1 expression via the activation of STING-IRF7 signal axis

GPATCH3, a protein with G-patch domain, is known to participate in innate immune response and organ development in mammals. However, there are few reports on GPATCH3 in fish. Here the cDNA sequence of GPATCH3 was cloned from Ctenopharyngodon idella (CiGPATCH3, MN149902) and was determined its character. A cDNA sequence of CiGPATCH3 is 1646 bp and contains an ORF of 1221 bp translating a protein of 407 amino acids. Phylogenetic analysis uncovered that CiGPATCH3 possesses a relatively high degree of homology with Cyprinus carpio GPATCH3. The mRNA level of CiGPATCH3 was increased following the intracellular stimulation of poly (I:C) into CIK cells. In vivo, over-expression of CiGPATCH3 can significantly up-regulate IFN 1 and ISG15 expression at mRNA and protein levels. To investigate the molecular mechanism by which GPATCH3 initiates the innate immune response in fish, co-IP experiments were performed to analyze the substrates of CiGPATCH3. The results showed that CiGPATCH3 directly interacted with CiSTING, but not with CiIRF3, CiIRF7, CiTBK1 or CiIPS-1. As compared with the single transfection of CO cells with either CiGPATCH3 or CiSTING, the expression of IFN 1 was more significantly up-regulated in cells under treatment with dual transfection of CiGPATCH3 and CiSTING. Knockdown of CiGPATCH3 inhibited STING-mediated IFN 1 expression in fish cells. Over-expression of CiGPATCH3 and CiSTING facilitated the phosphorylation and cytoplasmic-to-nuclear translocation of CiIRF7. These results explicitly showed that CiGPATCH3 up-regulates IFN 1 and ISG15 expression via the activation of STING-IRF7 signal axis in vivo.

Grass carp (Ctenopharyngodon idella) PACT induces cell apoptosis and activates NF-кB via PKR

As a dsRNA-dependent and interferon-induced protein kinase, PKR is involved in antiviral immune response and apoptosis mediated by various cytokines. In mammalian cells, PKR can also be activated in the absence of dsRNA. A PKR activator, PACT (PKR activating protein), also referred to as RAX (PKR-associated protein X) plays an important role. In recent years, with the increasing recognition of fish interferon system, PKR and PACT have been gradually revealed in fish. However, the function of fish PACT is unclear. In our previous work, we suggested that grass carp (Ctenopharyngodon idella) PACT must be involved in IRF2 and ATF4-mediated stress response pathways. In the present study, we found that the expression of C. idella PACT (CiPACT) and CiPKR were significantly up-regulated under the stimulation of LPS. It indicated that CiPACT and CiPKR may play an important role in response to LPS stimulation. In addition, the response time of CiPACT to LPS is earlier than that of CiPKR. It has also shown that overexpression of CiPACT in CIK cells can significantly enhance the level of p-eIF2α, induces apoptosis and translocation of Cip65 to nucleus from cytoplasm. To further understand the mechanism, we carried out the co-immunoprecipitation assay. It proved that the interaction of CiPACT and CiPKR made the phosphorylation of CiPKR. Overexpression of CiPACT induced the down-regulation of intracellular expression of bcl-2 and up-regulation of bax. However, in CiPKR knocked-down cells the expression of bcl-2 and bax were just the opposite. Therefore, the mechanism of fish PACT induces apoptosis and activates NF-кB is dependent on PKR.

PKZ, a Fish-Unique eIF2α Kinase Involved in Innate Immune Response

PKZ is a novel and unique eIF2α protein kinase identified in fish. Although PKZ is most homologous to PKR, particularly in the C-terminal catalytic domain, it contains two N-terminal Z-DNA-binding domains (Zα1 and Zα2) instead of the dsRNA binding domains (dsRBDs) in PKR. As a novel member of eIF2α kinase family, the available data suggest that PKZ has some distinct mechanisms for recognition, binding, and B-Z DNA transition. Functionally, PKZ seems to be activated by the binding of Zα to Z-DNA and participates in innate immune responses. In this review, we summarize the recent progress on fish PKZ.

Identification of the SAMHD1 gene in grass carp and its roles in inducing apoptosis and inhibiting GCRV proliferation

SAMHD1 is an innate immunity restriction factor that inhibits virus infection through IRF3-mediated antiviral and apoptotic responses. Fish SAMHD1 shares some similar properties with those in mammals. In this study, a SAMHD1 orthologue from grass carp (Ctenopharyngodon idellus) was cloned and characterized. The full-length cDNA of CiSAMHD1 is 2792 bp with an ORF of 1884 bp encoding a polypeptide of 627 amino acids. Multiple alignments showed that SAMHD1 is highly conserved among different species. Phylogenetic tree analysis revealed that CiSAMHD1 shared a high degree of homology with Sinocyclocheilus rhinocerous SAMHD1. Expression analysis indicated that CiSAMHD1 was widely expressed in all tissues tested including the brain, eyes, spleen, gill, intestine, liver, heart and kidney. It was significantly up-regulated in spleen, liver and intestines after treatment with poly I:C. Also, CiSAMHD1 can be induced following stimulation with recombinant IFN in CIK cells. The promoter sequence of CiSAMHD1 was identified to explore the mechanism underlying the transcriptional regulation of CiSAMHD1. The promoter sequence of CiSAMHD1 (1370 bp) consists of IRF1, IRF3, IRF9 and p65 binding elements. Gel mobility shift assay also showed that IRF1, IRF3, IRF9 and p65 prokaryotic proteins can separately interact with CiSAMHD1 promoter. Dual luciferase assay and q-PCR suggested that the promoter of CiSAMHD1 can be activated by the overexpression of CiIRF3 and CiIRF9, but cannot be triggered by CiIRF1 and Cip65. In contrast, knockdown of CiIRF3 or CiIRF9 inhibits the transcription of CiSAMHD1. Intriguingly, CCK assay suggested that CiSAMHD1 decreased cell viability. TUNEL apoptosis assay and Hoechst 33258 staining assay indicated that apoptosis is induced by the overexpression of CiSAMHD1. Crystal violet staining, detection of two GCRV genes (vp3 and vp5) and viral titration showed that CiSAMHD1 can suppress the proliferation of grass carp reovirus (GCRV) in CIK cells.

Identification and functional characterization of IRAK-4 in grass carp (Ctenopharyngodon idellus)

IL-1R-associated kinase 4 (IRAK4), a central TIR signaling mediator in innate immunity, can initiate a cascade of signaling events and lead to induction of inflammatory target gene expression eventually. In the present study, we cloned and characterized an IRAK4 orthologue from grass carp (Ctenopharyngodon idella). The full length cDNA of CiIRAK4 was 2057 bp with an ORF of 1422 bp encoding a polypeptide of 472 amino acids. Multiple alignments showed that IRAK4s were highly conserved among different species. Phylogenetic tree analysis revealed that CiIRAK4 shared high homologous with zebra fish IRAK4. Expression analysis indicated that CiIRAK4 was widely expressed in all tested tissues. It was significantly up-regulated after treatment with poly I:C, especially obvious in liver and spleen. Also, CiIRAK4 could be induced by poly I:C and LPS in CIK cells. Fluorescence microscopy assays showed that CiIRAK4 localized in the cytoplasm. RNAi-mediated knockdown and overexpression assays indicated that CiIRAK4 might have little effect on NF-kappa B p65 translocation from cytoplasm to nucleus, indicating that CiIRAK4 was dispensable for activation of NF-kappa B p65. In addition, IRAK4 promoted IRF5 nuclear translocation, which has nothing to do with the interaction between IRAK4 and IRF5. It suggested that fish IRAK4 kinase regulated IRF5 activity through indirect ways.

The Fish-Specific Protein Kinase (PKZ) Initiates Innate Immune Responses via IRF3- and ISGF3-Like Mediated Pathways

PKZ is a fish-specific protein kinase containing Zα domains. PKZ is known to induce apoptosis through phosphorylating eukaryotic initiation factor 2α kinase (eIF2α) in the same way as double-stranded RNA-dependent protein kinase (PKR), but its exact role in detecting pathogens remains to be fully elucidated. Herein, we have found that PKZ acts as a fish-specific DNA sensor by initiating IFN expression through IRF3- or ISGF3-like mediated pathways. The expression pattern of PKZ is similar to those of innate immunity mediators stimulated by poly (dA:dT) and poly (dG:dC). DNA-PKZ interaction can enhance PKZ phosphorylation and dimerization in vitro. These findings indicate that PKZ participates in cytoplasmic DNA-mediated signaling. Subcellular localization assays have also shown that PKZ is located in the cytoplasm, which suggests that PKZ acts as a cytoplasmic PRR. Meanwhile, co-IP assays have shown that PKZ can separately interact with IRF3, STING, ZDHHC1, eIF2α, IRF9, and STAT2. Further investigations have revealed that PKZ can activate IRF3 and STAT2; and that IRF3-dependent and ISGF3-like dependent mediators are critical for PKZ-induced IFN expression. These results demonstrate that PKZ acts as a special DNA pattern-recognition receptor, and that PKZ can trigger immune responses through IRF3-mediated or ISGF3-like mediated pathways in fish.

Fish SAMHD1 performs as an activator for IFN expression

As a host limiting factor, Sterile Alpha Motif and Histidine-Aspartate Domain 1 protein (SAMHD1) is associated with IRF3-mediated antiviral and apoptotic responses in mammals. However, the antiviral mechanism of SAMHD1 remains indistinct in fish. In this study, we found the expression of Ctenopharyngodon idella SAMHD1 (MF326081) was up-regulated after transfection with poly I:C (dsRNA analog), B-DNA or Z-DNA into C. idella kidney cells (CIKs), but these expression profiles had no obvious change when the cells were incubated with these nucleic acids. These data may indicate that CiSAMHD1 participates in the intracellular PRR-mediated signaling pathway rather than extracellular PRR-mediated signaling pathway. Subcellular localization assay suggested that a part of over-expressed CiSAMHD1 were translocated from nuclear to cytoplasm when C. idella ovary cells (COs) were transfected with poly I:C, B-DNA or Z-DNA. Nucleic acid pulldown assays were performed to investigate the reason for nuclear-cytoplasm translocation of CiSAMHD1. The results showed that CiSAMHD1 had a high affinity with B-DNA, Z-DNA and ISD-PS (dsRNA analog). In addition, co-IP assays revealed the interaction of CiSAMHD1 with CiSTING (KF494194). Taken together, all these results suggest that grass carp SAMHD1 performs as an activator for innate immune response through STING-mediated signaling pathway.

Ctenopharyngodon idella TBK1 activates innate immune response via IRF7

In mammals, IFN regulatory factor (IRF) 7 is a central regulator of IFN-α expression in response to variable pathogenic infections. There are several pathogenic sensors involved in monitoring pathogen intrusion in mammals. These sensors trigger IRF7-mediated responses through different pathways. TANK-binding kinase 1 (TBK1) is a critical mediator of IRF7 activation upon pathogen infection. In fish, there are many reports on TBK1, IRF3 and IRF7, especially on TBK1-IRF3 signaling pathway. However, it is not very clear how TBK1-IRF7 works in innate immune signaling pathway. In this study, we explored how TBK1 up-regulates IFN, ISG expression, and how TBK1 initiates innate immune response through IRF7 in fish under lipopolysaccharides (LPS) stimulation. After stimulation with LPS, grass carp IRF3 and IRF7 transcriptions were up-regulated, indicating they participate in TLR-mediated antiviral signaling pathway. It is interesting that the response time of grass carp IRF3 to LPS was earlier than that of IRF7. In addition, IRF7 rather than IRF3 acted as a stronger positive regulator of IFN and ISG transcription in Ctenopharyngodon idella kidney cells (CIKs). It is suggested the potential function differentiation between IRF3 and IRF7 upon LPS infection in fish. Dual luciferase assays also showed that overexpression of grass carp IRF7 and TBK1 up-regulated the transcription level of IFN and PKR. However, knockdown of IRF7 inhibits ISG expression, suggesting that grass carp TBK1 regulates the transcription via IRF7. Co-immunoprecipitation and GST pull-down assays proved the binding of grass carp IRF7 to TBK1. Furthermore, grass carp TBK1 can promote the nuclear translocation of IRF7. The results indicated that grass carp TBK1 can bind directly to and activate IRF7.

Endoplasmic Reticulum Transmembrane Proteins ZDHHC1 and STING Both Act as Direct Adaptors for IRF3 Activation in Teleost

IFN regulatory factor (IRF)3 is a central regulator for IFN-β expression in different types of pathogenic infections. Mammals have various pathogenic sensors that are involved in monitoring pathogen intrusions. These sensors can trigger IRF3-mediated antiviral responses through different pathways. Endoplasmic reticulum-associated proteins stimulator of IFN gene (STING) and zinc finger DHHC-type containing 1 (ZDHHC1) are critical mediators of IRF3 activation in response to viral DNA infections. In this study, grass carp STING and ZDHHC1 were found to have some similar molecular features and subcellular localization, and both were upregulated upon stimulation with polyinosinic:polycytidylic acid, B-DNA, or Z-DNA. Based on these results, we suggest that grass carp STING and ZDHHC1 might possess some properties similar to their mammalian counterparts. Overexpression of ZDHHC1 and STING in Ctenopharyngodon idella kidney cells upregulated IFN expression, whereas knockdown of IRF3 inhibited IFN activation. In addition, coimmunoprecipitation and GST pull-down assays demonstrated that STING and ZDHHC1 can interact separately with IRF3 and promote the dimerization and nuclear translocation of IRF3. Furthermore, we also found that small interfering RNA-mediated knockdown of STING could inhibit the expression of IFN and ZDHHC1 in fish cells. Similarly, knockdown of STING resulted in inhibition of the IFN promoter. In contrast, ZDHHC1 knockdown also inhibited IFN expression but had no apparent effect on STING, which indicates that STING is necessary for IFN activation through ZDHHC1. In conclusion, STING and ZDHHC1 in fish can respond to viral DNA or RNA molecules in cytoplasm, as well as activate IRF3 and, eventually, trigger IFN expression.

Ctenopharyngodon idella IKKβ interacts with PKR and IκBα

Inhibitor of nuclear factor kappa-B kinase β (IKKβ) is a subunit of the IKK complex. It can activate the NF-κB pathway through phosphorylating IκB in response to a wide range of stimuli. In the present study, an IKKβ gene from grass carp (Ctenopharyngodon idella; KT282114) was cloned and identified by homologous cloning and rapid-amplification of cDNA ends (RACE) technique. The complete CiIKKβ cDNA is 3428 bp in length, with the longest open reading frame (ORF) of 2337 bp encoding a polypeptide of 778 amino acids. The deduced amino acid sequence of CiIKKβ has similar domain distribution to those of mammalian. For example, CiIKKβ consists of a serine/threonine kinase domain at the N-terminal, a basic region leucin zipper (BRLZ) domain in the middle, a homeobox associated leucin zipper (HALZ) domain and an IKKβ NEMO (NF-κB essential modulator) binding domain at the C-terminal. Phylogenetic tree analysis also showed that CiIKKβ is highly homologous to zebrafish IKKβ (DrIKKβ) and clearly distinct from the mammalian and amphibian counterparts. The expression of CiIKKβ was ubiquitously found in the liver, intestine, kidney, gill, spleen, heart, and brain tissues of grass carp and significantly up-regulated in CIK cells under the stimulation with Poly I:C and UV-inactivated grass carp hemorrhagic virus. To investigate the activation mechanism of NF-κB pathway in fish and the role of CiIKKβ in the pathway, we explored the protein interactions of protein kinase R (PKR) with IKKβ and IKKβ with IκBα by co-immunoprecipitation and GST-pull down assays. The interaction between each pair was confirmed. The results suggest that CiIKKβ may be a primary member in the activation of NF-κB pathway in fish.