RIPK3 collaborates with RIPK1 to inhibit MAVS-mediated signaling during black carp antiviral innate immunity

The Evolution and Biological Activity of Metazoan Mixed Lineage Kinase Domain-Like Protein (MLKL)

In mammals, mixed lineage kinase domain-like protein (MLKL) is the executor of necroptosis. MLKL comprises an N-terminal domain (NTD), which alone suffices to trigger necroptosis by forming pores in the plasma membrane, and a C-terminal domain that inhibits the NTD activity. Evolutionarily, MLKL is poorly conserved in animals and not found in Protostomia. Although MLKL orthologs exist in invertebrate Deuterostomia, the biological activity of invertebrate MLKL is unknown. Herein, we examined 34 metazoan phyla and detected MLKL not only in Deuterostomia but also in Protostomia (Rotifera). The Rotifera MLKL exhibited low identities with non-Rotifera MLKL but shared relatively high identities with non-metazoan MLKL. In invertebrates, MLKL formed two phylogenetic clades, one of which was represented by Rotifera. In vertebrates, MLKL expression was tissue-specific and generally rich in immune organs. When expressed in human cells, the MLKL-NTD of Rotifera, Echinodermata, Urochordata, and Cephalochordata induced strong necroptosis. The necroptotic activity of Rotifera MLKL depended on a number of conserved residues. Together these findings provided new insights into the evolution of MLKL in Metazoa and revealed the biological activity of invertebrate MLKL.

ATG16L1 negatively regulates MAVS-mediated antiviral signaling in black carp Mylopharyngodon piceus

Autophagy related 16 like 1 (ATG16L1) is a crucial component of autophagy that regulates the formation of the autophagosome. In mammals, ATG16L1 also performs important roles in immunity, including controlling viral replication and regulating innate immune signaling; however, investigation on the role of piscine ATG16L1 in immunity is rare. In this report, the ATG16L1 homolog of black carp Mylopharyngodon piceus (bcATG16L1) was cloned and identified, and its negative regulatory role in mitochondrial antiviral signaling protein (MAVS)-mediated antiviral signaling was described. The coding region of bcATG16L1 consists of 1830 nucleotides and encodes 609 amino acids, including one coiled-coil domain at the N-terminus, three low complexity region domains in the middle, and seven WD40 domains at the C-terminus. By immunofluorescence assay and immunoblotting, we found that bcATG16L1 is a cytosolic protein with a molecular weight of ∼74 kDa. In addition, over-expression of bcATG16L1 suppressed bcMAVS-mediated bcIFNa and DrIFNφ1 promoters transcriptional activity and inhibited bcMAVS-mediated antiviral activity. We further confirmed the co-localization of bcATG16L1 and bcMAVS by immunofluorescence assay and verified the protein interaction between bcATG16L1 and bcMAVS by immunoprecipitation assay. Our results report for the first time that black carp ATG16L1 suppresses MAVS-mediated antiviral signaling in teleost fish.

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.

RIP3 Associates with RIP1, TRIF, MAVS, and Also IRF3/7 in Host Innate Immune Signaling in Large Yellow Croaker Larimichthys crocea

Receptor-interacting protein 3 (RIP3) has been demonstrated to be a key regulator not only in cell death pathways including apoptosis and necroptosis but also in inflammation and host immune responses. In this study, a RIP3 ortholog named Lc-RIP3 is identified in large yellow croaker (Larimichthys crocea). The open reading frame (ORF) of Lc-RIP3 is 1524 bp long and encodes a protein of 507 amino acids (aa). The deduced Lc-RIP3 protein has an N-terminal kinase domain and a C-terminal RHIM domain, and the genome organization of Lc-RIP3 is conserved in teleosts with 12 exons and 11 introns but is different from that in mammals, which comprises 10 exons and 9 introns. Confocal microscopy revealed that Lc-RIP3 is a cytosolic protein. The expression analysis at the mRNA level indicated that Lc-RIP3 is ubiquitously distributed in various tissues/organs, and could be up-regulated under poly I:C, LPS, PGN, and Pseudomonas plecoglossicida stimulation in vivo. Notably, Lc-RIP3 could induce NF-κB but not IRF3 activation. In addition, Lc-RIP3 co-expression with Lc-TRIF, Lc-MAVS, or Lc-IRF3 significantly abolishes the activation of NF-κB but enhances the induction of IRF3 activity. Moreover, NF-κB activity could be up-regulated when Lc-RIP3 is co-expressed with Lc-RIP1 or Lc-IRF7. These results collectively indicate that Lc-RIP3 acts as an important regulator in host innate immune signaling in teleosts.