Ctenopharyngodon idella IRF2 and ATF4 down-regulate the transcriptional level of PRKRA

Basal and immune-responsive transcript expression of two Atlantic salmon interferon regulatory factor 2 (irf2) paralogues

Atlantic salmon (Salmo salar) is one of the most economically important aquaculture species globally. However, disease has become a prevalent threat to this industry. A thorough understanding of the genes and molecular pathways involved in the immune responses of Atlantic salmon is imperative for selective breeding of disease-resistant broodstock, as well as developing new diets and vaccines to mitigate the impact of disease. Members of the interferon regulatory factor (IRF) family of transcription factors play roles in the induction of interferons and other cytokines involved in host immune responses to intracellular and parasitic pathogens. IRF family members also play diverse roles in other biological processes, such as stress response, reproduction and development. The current study focused on one member of the IRF family: interferon regulatory factor 2 (irf2). As previously shown, due to the genome duplication that occurred ∼80 million years ago in the salmonid lineage, there are two irf2 paralogues in the Atlantic salmon genome. In silico analyses at the cDNA and deduced amino acid levels were conducted followed by phylogenetic tree construction with IRF2 amino acid sequences from various ray-finned fishes, cartilaginous fish and tetrapods. qPCR was then used to analyze paralogue-specific irf2 constitutive expression across 17 adult tissues, as well as responses to the viral mimic pIC (i.e., synthetic double-stranded RNA analog) in cultured macrophage-like cells (in vitro) and to infection with the Gram-negative bacterium Moritella viscosa in skin samples (in vivo). The qPCR studies showed sex- and paralogue-specific differences in expression across tissues. For example, expression of both paralogues was higher in ovary than in testes; expression (considering both sexes together) was highest for irf2-1 in gonad and for irf2-2 in hindgut. Both irf2 paralogues were responsive to pIC stimulation, but varied in their induction level, with irf2-1 having an overall stronger response than irf2-2. Only one paralogue, irf2-2, was significantly responsive to M. viscosa infection. Differences in irf2-1 and irf2-2 transcript expression levels constitutively across tissues, and in response to pIC and M. viscosa, may suggest neo- or subfunctionalization of the duplicated genes. This novel information expands current knowledge and provides insight into how genome duplication events may impact host regulation of important immune markers.

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.

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.

Grass carp (Ctenopharyngodon idella) Bcl-xl: transcriptional regulation and anti-apoptosis analysis

Bcl-xl, Bax2, and NF-κB are well-known to be involved in anti-apoptosis response. Although Bcl-xl has been reported in fish, the NF-κB-mediated regulatory mechanism and anti-apoptotic function are still unclear. Here, we cloned and characterized the full-length cDNA sequence of grass carp (Ctenopharyngodon idella) Bcl-xl (CiBcl-xl) and its promoter region sequence. The full-length cDNA of CiBcl-xl is 2836 bp with an ORF of 627 bp encoding a polypeptide of 208 amino acids. Phylogenetic tree analysis revealed that CiBcl-xl shared high homology with Dario rerio Bcl-xl (DrBcl-xl). After stimulation with Poly I:C, the expression of CiBcl-xl in CIK cells and various tested tissues of grass carp were significantly upregulated. To further understand the transcriptional control of fish Bcl-xl induced by NF-κB, CiC-rel and Cip65 were expressed in Escherichia coli BL21 and purified by affinity chromatography with the Ni-NTA His-Bind resin. In vitro, gel mobility shift assays demonstrated the high affinity of CiC-rel and Cip65 with CiBcl-xl promoter. Dual-luciferase reporter assays showed that CiC-rel and Cip65 activated CiBcl-xl promoter. Also, knockdown of CiC-rel and Cip65 reduced the expression of Bcl-xl. Therefore, similar to those of mammals, fish C-rel and p65 can upregulate the transcription of Bcl-xl. In addition, we found that overexpression of CiBcl-xl in CIK cells increased the cell activity and inhibited cell apoptosis, while overexpression of Bax2 promoted cell apoptosis. Meanwhile, co-transfection of CiBcl-xl and CiBax2 into cells can ease up apoptotic rate. To further investigate the molecular basis of synergistic effect of Bcl-xl and Bax2, we showed that Bcl-xl and Bax2 interacted with each other. The results suggested that Bcl-xl executed its anti-apoptotic function by binding to and inhibiting the pro-apoptotic activity of Bax2.

Grass carp (Ctenopharyngodon idellus) TRAF6 up-regulates IFN1 expression by activating IRF5

In mammals, interferon regulatory factor 5 (IRF5) can be activated by tumor necrosis factor receptor-associated factor 6 (TRAF6). Upon activation, IRF5 translocates into the nucleus, where it binds to IFN promoter and up-regulates IFN expression. However, there are few reports on the molecular mechanism by which TRAF6 up-regulates IFN expression in fish. In this study, we explored how Grass carp (Ctenopharyngodon idellus) TRAF6 initiated innate immunity by activating IRF5. We found that CiTRAF6, CiIRF5 and CiIFN1 were all significantly up-regulated in LPS-stimulated CIK cells and the expression of CiTRAF6 was earlier than the expressions of CiIRF5 and CiIFN1. These findings suggested that CiIFN1 expression might be induced by CiTRAF6 in fish. CiIFN1 expression, CiIFN1 promoter activity and CO cells viability were all significantly up-regulated in the overexpression experiments, but they were significantly down-regulated in the gene silencing experiments. This indicated that CiTRAF6, along with CiIRF5, regulated CiIFN1 expression. The localization analysis found that both CiTRAF6 and CiIRF5 located in the cytoplasm. Following LPS stimulation, CiIRF5 was observed to translocate to the nucleus. GST-pull down and co-IP experiments revealed that CiTRAF6 interacted with CiIRF5. The colocalization analysis also showed that CiTRAF6 bound with CiIRF5 in the cytoplasm. Overexpression of CiTRAF6 increased the endogenous CiIRF5, promoted its ubiquitination and nuclear translocation. In conclusion, CiTRAF6 bound to CiIRF5 in the cytoplasm, and then activated CiIRF5, resulting in up-regulating the expression of CiIFN1.