Zebrafish (<i>Danio rerio</i>) TDP43 negatively regulates PKZ-IRF3–mediated IFN I response

Transactive response DNA binding protein 43 kD (TDP43), encoded by the tardbp gene, is a member of heterogenous nuclear ribonucleoproteins family. In this study, a gradual upregulation of TDP43 messenger RNA was observed in either Ctenopharyngodon idella kidney cells or zebrafish following stimulation with B-DNA, grass carp reovirus, or spring viremia of carp virus. Moreover, grass carp reovirus stimulation enhances the dimerization, phosphorylation, and cytoplasm-to-nucleus translocation of TDP43 in zebrafish (DrTDP43). Type I interferon (IFN I) expression is inhibited in a dose-dependent manner in the cells transfected with DrTDP43 under GCRV stimulation. These results indicated that DrTDP43 is involved in innate immune response and serves as a negative regulator of IFN I expression. To determine DrTDP43-dependent downstream pathway in innate immunity, the substrate of DrTDP43 was studied. It is known that IFN I expression can be activated by PKZ via IRF3 dependent pathway. Our results found that DrTDP43 can be interacted with PKZ, suggesting that the downregulation of IFN I by DrTDP43 may attribute to the inhibition of PKZ activity. Multiple DrTDP43 mutants were constructed to further reveal the mechanism of TDP43-PKZ-mediated IFN I response. Apart from the N-terminal domain, RNA recognition motif 1, RNA recognition motif 2, and low-complexity domain domains of DrTDP43 were all found to be involved in inhibiting phosphorylation of PKZ. In vivo, knockdown of TDP43 in zebrafish embryos improved embryo survival rate upon viral infection and upregulated expression of IFN I. In summary, our findings demonstrate that DrTDP43 is a negative regulator of IFN I expression through the inhibition of the PKZ-IRF3-dependent pathway.