Orange-spotted grouper nervous necrosis virus-encoded protein A induces interferon expression via RIG-I/MDA5-MAVS-TBK1-IRF3 signaling in fish cells
Authors: Huang Siyou , Huang Yi , Su Taowen , Huang Runqing , Su Lianpan , Wu Yujia , Weng Shaoping , He Jianguo , Xie Junfeng 
Published: December 2023

ABSTRACT
Nervous necrosis virus (NNV), a highly contagious fish virus, has caused huge economic losses to the global aquaculture industry. A previous study showed that protein A (ProA) encoded by orange-spotted grouper NNV triggers type I interferon (IFN) production in fish cells, but the activation and modulation of correlative signal pathways remain unclear. Here, we proved that ProA induces fish cell-specific IFN promoter activation in a dose-dependent manner. In channel catfish ovary (CCO, an NNV-permissive cell), ProA evoked expression and secretion of functional IFN with anti-DNA virus activity, suggesting a good model for IFN signaling research. A contrastive study of signaling in CCO and fathead minnow (FHM, an NNV-nonpermissive cell) using RNAi knockdown or dominant negative mutant overexpression verified that ProA-mediated IFN activation went through retinoic acid-inducible gene I (RIG-I)-like receptor (RLR) pathway (RIG-I/MDA5-MAVS-TRAF3-TBK1-IRF3) while NOD1-RIPK2-NFκB and TLR3-TRIF branches were unnecessary. As RNA receptors upregulated by ProA in FHM, RIG-I and MDA5 promoted ProA-mediated IFN activation to form a positive feedback loop, while LGP2, NOD1, and PKR inhibited this activation as negative modulators. In ProA-expressed, NNV-infected CCO, the transcription of RIG-I, MDA5, MAVS, TBK1, IRF3, and IFN was downregulated, but the expression of LGP2, TRAF3, and NOD1 remained unchanged, suggesting unknown IFN suppression mechanism by NNV infection. IFN inhibition by overexpressing mutated RIG-I greatly enhanced NNV replication in FHM, implying that RIG-I might be the main target for both ProA-mediated activation and NNV infection-induced inhibition. This study provides overviews and foundations for understanding the interaction between betanodavirus-encoded protein and fish innate immune signaling.

IMPORTANCE
As a major pathogen, nervous necrosis virus (NNV) infects more than 120 fish species worldwide and is virulent to larvae and juvenile fish, hampering the development of the fish fry industry. Understanding virus-host interaction and underlying mechanisms is an important but largely unknown issue in fish virus studies. Here, using channel catfish ovary and fathead minnow cells as models for the study of innate immunity signaling, we found that NNV-encoded ProA activated interferon signaling via the retinoic acid-inducible gene I (RIG-I)-like receptor (RLR) pathway which was still suppressed by the infection of wild-type NNV. This finding has important implications for the comprehension of NNV protein function and the immune response from different cells. First, RIG-I is the key node for anti-NNV innate immunity. Second, the response intensity of RLR signaling determines the degree of NNV proliferation. This study expands our knowledge regarding the overview of signal pathways affected by NNV-encoded protein and also highlights potential directions for the control of aquatic viruses.