MicroRNA-203-3p participates in antiviral immune response by negatively regulating TRAF3 in the rainbow trout (Oncorhynchus mykiss).

MicroRNAs (miRNAs) are highly conserved endogenous non-coding RNAs that play a crucial role in fish immune response by regulating gene expression at the post-transcriptional level. In recent years, the viral diseases caused by infectious hematopoietic necrosis virus (IHNV) have caused significant economic losses in rainbow trout (Oncorhynchus mykiss) aquaculture, whereas the immune regulatory mechanisms of miRNAs involved in rainbow trout resistance to IHNV infection remains largely undefined. In this study, we analyzed the structural characteristics of Oncorhynchus mykiss tumor necrosis factor receptor-associated factor 3 (OmTRAF3) by bioinformatics software and explored the molecular mechanism of miR-203-3p in rainbow trout resistance to IHNV by regulating OmTRAF3 in vivo and in vitro. The open reading frame (ORF) of OmTRAF3 gene was 1731 bp and encoded 576 amino acids including an N-terminal RING finger domain, two zinc finger domains, a coiled-coil domain, and a C-terminal MATH domain. The expression pattern analysis showed that the expression of miR-203-3p and OmTRAF3 in immune-related tissues (head kidney, spleen, and liver) and liver cells of rainbow trout infected with IHNV varied with certain regularity and had opposite trends at key time points, and a targeting relationship between miR-203-3p and OmTRAF3 was confirmed using a dual luciferase reporter gene assay. Further, we found that in vivo and in vitro overexpression of miR-203-3p significantly reduced the expression of OmTRAF3, downstream immune-related genes (OmTANK, OmIKKε, OmIFN1, and OmISG15) and promoted IHNV copy number replication, while silencing of miR-203-3p yielded opposite results. More importantly, OmTRAF3 and downstream genes as well as IHNV copy number changed accordingly with the silencing of OmTRAF3. The above results revealed that miR-203-3p participates in the immune response against IHNV by targeting OmTRAF3, and provides a theoretical basis for the screening of antiviral drugs in rainbow trout.