SiRNA-mediated knockdown of CiGRP78 gene expression leads cell susceptibility to heavy metal cytotoxicity

Heavy metal toxicity leads to accumulation of insoluble proteins and induces endoplasmic reticulum stress-specific unfolded protein response in Arabidopsis thaliana

Unfolded protein accumulation in the endoplasmic reticulum (ER) triggers ER stress, leading to a unique transcriptomic response called unfolded protein response (UPR). While ER stress is linked to various environmental stresses, its role in plant responses to heavy metal toxicity remains unclear. This study aimed to elucidate if heavy metals Fe, Zn, Cu, and As induce ER stress in plants. For this purpose, Arabidopsis thaliana seedlings were treated with Fe (200, 400 µM), Zn (500, 700 µM), Cu (25, 50 µM), and As (250, 500 µM) for 7 days, which resulted in 50-70% decrease in plant growth. All treatments increased insoluble protein levels, indicating unfolded protein accumulation, with the highest induction observed for 50 µM Cu treatment (fivefold). Expressions of genes involved in the perception and signaling of ER stress (IRE1, bZIP28, bZIP60, bZIP17) indicate that Zn toxicity specifically induces bZIP28 but not the IRE1 branch of UPR. All metals except Fe also induced genes associated with protein folding in the ER (BIP1, BIP3, and CNX) and ER-associated protein degradation (ERAD) (HRD1). This finding indicates Zn, Cu, and As but not Fe cause ER stress in plants. Furthermore, increased expression of ER oxidoreductase 1 (ERO1) suggests that metal toxicity also disrupts oxidative protein folding in the ER lumen. This study enhances our understanding of the intricate interplay between essential nutrients, metal toxicity, protein folding machinery, and ER stress, demonstrating that heavy metal toxicity has an ER stress component in plants alongside its established effects on energy metabolism, membrane integrity, and oxidative stress.

Grass carp (Ctenopharyngodon idella) NLK2 inhibits IFN I response through blocking MAVS-IRF3 axis

In mammals, nemo-like kinase 2 (NLK2) is a conservative protein kinase involved in Wnt/β-catenin signaling pathway and immune response. However, the role of NLK2 in immune response in teleost remain unclear. In this study, we identified an ortholog of mammalian NLK from grass carp (Ctenopharyngodon idellus) named CiNLK2. CiNLK2 shares a high level of homology with the counterparts, especially with that of Cyprinus carpio. CiNLK2 was ubiquitously expressed in all tested tissues (liver, brain, spleen, gill, kidney and eye) and its expression was up-regulated under the treatment with poly I:C or GCRV. Overexpression of CiNLK2 suppressed the production of IFN I in CIK cells whether or not treated with poly I:C. However, knockdown of CiNLK2 increased the expression level of IFN I. The analysis of subcellular localization showed that CiNLK2 protein was scattered throughout the cytoplasm and nucleus. In terms of mechanism, CiNLK2 can directly interact with MAVS and inhibit MAVS-induced IFN I response. Moreover, CiNLK2 increased the phosphorylation level of MAVS, which led to the degradation of MAVS protein. On the other hand, CiNLK2 suppressed the phosphorylation and nuclear translocation of IRF3. In general, CiNLK2 served as an inhibitor for IFN I response by targeting MAVS-IRF3 signal 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.

Grass carp Mre11A activates IFN 1 response by targeting STING to defend against GCRV infection

Mre11A is considered as a cytosolic DNA receptor in mammals. However, it is rarely known about Mre11A in other vertebrates. Recently, a mammalian ortholog of Mre11A has been identified in grass carp (Ctenopharyngodon idellus) in our lab. Phylogenetic-tree analysis provided evidence for a close genetic relationship between C.idellus Mre11A and Carassius auratus Mre11A. The tissue expression profile of CiMre11A was detected, with a relatively higher level of expression in kidney, intestines, liver and spleen than that in other tissues after grass carp reovirus (GCRV) infection. Similarly, CiMre11A was also up-regulated in CIK cells after treatment with GCRV. Q-PCR and dual-luciferase assays indicated that the transcription levels of IFN1 and ISG15 were inhibited by CiMre11A knockdown, but were gradually augmented after CIK cells were transfected with increasing amounts of CiMre11A. Subcellular localization assays showed that a part of CiMre11A was translocated from the nucleus to the cytoplasm. Co-immunoprecipitation and co-localization assays demonstrated that CiMre11A interacts with CiSTING in response to GCRV infection. In CIK cells, the expressions of both IFN1 and ISG15 were acutely up-regulated by CiMre11A overexpression, as well as by co-overexpression of CiMre11A and CiSTING. CiMre11A and CiSTING induced the phosphorylation and cytoplasmic-to-nuclear translocation of IRF7 in CIK cells. The multiplication of GCRV in CIK cells was inhibited by the overexpression of CiMre11A and CiSTING.

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.

Ctenopharyngodon idellus DDX41 initiates IFN I and ISG15 expression in response to GCRV infection

As a member of DExD/H-box helicase family, DDX41 (DEAD box polypeptide 41) acts as an intracellular DNA sensor that induces type I IFN expression in mammals. Fish DDX41 shares some similar properties with the mammalian counterparts. In this study, a DDX41 orthologous gene from grass carp (Ctenopharyngodon idellus) (CiDDX41) was cloned and characterized. The ORF of CiDDX41 encodes a polypeptide of 614 amino acids. Multiple alignments showed that DDX41 is highly conserved among different species. Phylogenetic tree analysis revealed that CiDDX41 shares a high degree of homology with Sinocyclocheilus rhinocerous DDX41. CiDDX41 is highly expressed in kidney, intestines, liver and spleen. Their expressions are up-regulated more obviously after the treatment with GCRV. Over-expression of CiDDX41 in CIK cells increases the transcription level of grass carp IFN I and ISG15. On the contrary, knockdown of CiDDX41 inhibits the IFN I and ISG15 transcription. Moreover, a part of CiDDX41 translocates from the nuclear to cytoplasm to interact with grass carp STING (CiSTING). In CIK cells, overexpression of CiDDX41 and CiSTING can promote the phosphorylation and nuclear-cytoplasm translocation of grass carp IRF7 (CiIRF7) and then acutely up-regulate the IFN I and ISG15 expression. However, the knockdown of CiDDX41 inhibits the phosphorylation IRF7. Taken together, all these results above suggested that CiDDX41 performs as an activator for innate immune through STING-IRF7 mediated signaling pathway.

Overexpression of CiIKKβ enhances CIK cell viability against ER stress

Recently, studies have shown that IκB kinase β (IKKβ), a critical kinase in the nucleus factor kappa-B (NF-κB) pathway, participates in inflammatory responses associated with unfolded protein response (UPR) and plays an important role in ER stress-induced cell death. The unfolded protein response (UPR), which is a regulatory system to restore cellular homeostasis in the endoplasmic reticulum (ER), such as oxidative stress, bacterial infection, and virus invasion. The UPR pathways have been reported to be involved in immune responses in mammals, including the classical NF-κB pathway. However, the molecular mechanism of their crosstalk remains to be elucidated. Previously, we demonstrated that IKKβ also has some conserved functions between fish and human, as grass carp (Ctenopharyngodon idella) IKKβ (CiIKKβ) can activate NF-κB pathway. In this study, we found that CiIKKβ level in nucleus was elevated under ER stress and CiIKKβ can interact with grass carp X-box-binding protein 1 (CiXBP1S), a key transcription factor in UPR. Consistently, fluorescent histochemical analysis of grass carp kidney (CIK) cells indicated that CiIKKβ and CiXBP1S colocalized under ER stress. Furthermore, overexpression of CiIKKβ in CIK cells enhanced ER stress tolerance by regulating UPR signaling and resulted in the significant increase of cell viability.

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.

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.

A mechanism underlies fish GRP78 protection against Pb2+ toxicity

Heavy metal exposure impacts basic cellular processes and results in serious toxicological effects. Pb²⁺ can activate the response to endoplasmic reticulum (ER) stress by protein denaturation, changing intracellular calcium homeostasis, and inducing cell death. As an ER retention protein, 78-kDa glucose-regulated protein (GRP78) can relieve the Pb²⁺-induced ER stress and enhance cell viability. We previously showed that heavy metal ions such as Pb²⁺ etc. are harmful to fish cell lines in a time- and dose-dependent manner. The phenomenon is accompanied by the increasing accumulation of grass carp GRP78 (CiGRP78), which can protect the cells from heavy metal ion cytotoxicity. Here, we investigated the mechanism in which CiGRP78 exerted its protective function. Using metal ions affinity elution method and fluorescent spectral analysis, we showed that CiGRP78 could respectively form a complex with Calcium, Lead and Cadmium ions, especially with Lead ion in vitro. However, another ER retention protein CiGRP94 could not bind to Pb²⁺, highlighting the functional differentiation might exist in CiGRP78 and CiGRP94 in regulating heavy metal cytotoxicity. Our results suggested that CiGRP78 might increase cellular tolerance to Pb²⁺ via the direct interaction with it.

Using a multi-biomarker approach to assess the effects of pollution on sand flathead (Platycephalus bassensis) from Port Phillip Bay, Victoria, Australia

Hepatic gene expression and liver histology were examined in sand flathead (Platycephalus bassensis) from six locations in Port Phillip Bay, Victoria, Australia. Four sets of genes including thyroid-related genes (D1, D2, TTR, TRα and TRβ), metal metabolism-related genes (MT, MTF1, TF, Ferritin and FPN1), apoptosis-related genes (Diablo/SMAC1, Diablo/SMAC2 and CYP1A) and an endoplasmic reticulum stress biomarker gene (GRP78) were examined in female flathead using qRT-PCR. TRβ and Diablo/SMAC1 gene expression was significantly up-regulated in fish from all polluted sites compared to those from a reference site. The transcripts of TRα and FPN1 were significantly higher in flathead from Corio Bay, while the hepatic mRNA of TTR and GRP78 were significantly lower in those fish. Positive correlations were observed between Diablo/SMAC1 and CYP1A, D2 and TRβ, TRα and TRβ. This study demonstrates that application of pathway-based biomarker genes and histopathology can provide comprehensive information on the impact of environmental pollutants on fish.