Transcriptional profiling of receptor-transporting protein 3 variants in European sea bass and gilthead seabream suggests divergent roles.

Abstract

In marine aquaculture, red grouper nervous necrosis virus (RGNNV) poses a major threat to species such as European sea bass (Dicentrarchus labrax), while others like gilthead seabream (Sparus aurata) show asymptomatic resistance. The receptor-transporting protein 3 (rtp3) gene is known to be virus-responsive in fish, but its functional relevance in antiviral defense remains poorly understood. This study investigates the transcriptional divergence of the rtp3x1 and rtp3x2 paralogues in both fish species to explore their potential role in species-specific immune strategies. Phylogenetic analysis of protein sequences revealed that RTP3 X1 and X2 originated from an ancient duplication and form distinct clades, both conserving three CxxC motifs associated with antiviral functions in vertebrates. Structural modeling showed that RTP3 X1 is highly conserved across species, whereas RTP3 X2 exhibits greater divergence in interface regions, suggesting functional specialization. Expression profiling during development and in juvenile tissues uncovered variant- and host-specific regulation. In European sea bass, rtp3x1 exhibited transcriptional maternal inheritance with later gene expression reactivation. rtp3x2 was highly inducible and responsive to immune stimulation, with strong up-regulation following T-cell mitogens, viral mimics, RGNNV exposure, as well as in cell-mediated cytotoxicity (CMC) assays using RGNNV-infected target cells. In contrast, gilthead seabream showed consistently high basal expression of both variants throughout development, particularly in immune-related tissues, displaying either unmodulated or down-regulated expression under equal in vitro conditions. These transcriptional patterns were further supported in vivo: RGNNV challenge sharply induced rtp3 variants' expression in the brain of European sea bass, particularly rtp3x2, while gilthead seabream maintained stable transcript levels of both variants despite persistent infection. These findings support a model of post-duplication functional divergence, where RTP3 variants might contribute to an inducible CMC response in European sea bass, while gilthead seabream relies on constitutive expression potentially linked to antiviral tolerance. This divergence may explain species-specific susceptibility to RGNNV and highlights rtp3x1 and rtp3x2 paralogues as modulators of immune strategies in teleost fish.