Functional assessment of European sea bass RTP3 proteins and their effect on RGNNV replication

RTP3 proteins seem to play a prominent role in the innate immune response of European sea bass against NNV infections by still unknown mechanisms. The current work is a comprehensive study on the anti-RGNNV activity of European sea bass RTP3 proteins (RTP3 X1 and RTP3 X2), exploring several possible mechanisms of action. As a starting point, sea bass rtp3 X1 and rtp3 X2 ORFs were cloned in expression vectors and transfected in the E-11 cell line to obtain cells stably expressing RTP3 X1 and/or RTP3 X2. The analyses conducted with these cells have demonstrated that RTP3 X2 is an antiviral protein, whereas RTP3 X1 has a pro-viral effect. The functional evaluation has revealed that both proteins (alone or co-expressed) abolished the induction of mx and tnf-α genes, and triggered an enhanced cellular tolerance to oxidative stress, which was remarkably high in RTP3 X1-expressing cells. In addition, only RTP3 X1 promoted apoptosis in non-infected cells. Therefore, apoptosis and oxidative stress tolerance can be involved in the pro-viral effect exerted by RTP3 X1. These results have demonstrated that sea bass RTP3 X1 and RTP3 X2 are versatile proteins that may influence multiple cellular functions, and provide the basis for further studies to understand their mechanisms of action.