TASTE MATTERS? MOLECULAR CHARACTERIZATION OF TASTE RECEPTOR FAMILY TAS1R IN SEABREAM (SPARUS AURATA) TOWARDS FUTURE FUNCTIONAL STUDIES

Over the past decade, sustainable aquaculture production promoting increasing dietary replacement of fisheries by plant ingredients has steadily grown, often reducing the palatability of fish diets and animal performance. However, very little information exists on taste-related mechanisms possibly underlying these effects. Vertebrate taste perception is largely controlled by two families of G-protein-coupled receptors, taste receptor 1 (TAS1R) and taste receptor 2 (TAS2R), involved in recognition of sweet/umami and bitter tastes, respectively. Tastant selectivity through the TAS1R family is determined by the nature of combinatorial arrangement of TAS1R1, TAS1R2 and TAS1R3 heterodimeric subunits. The mammalian TAS1R1/TAS1R3 receptor responds to umami compounds such as amino acids, whereas TAS1R2/TAS1R3 is activated by sweet substances. Nevertheless, in some teleost fish, TAS1R2 duplicated genes revealed species-specific functional plasticity with a prominent response to amino acid ligands. In order to investigate TAS1R functional properties and their potential distinct evolutionary trajectories in fish, we cloned the full-length cDNA of five TAS1R genes in Sparus aurata (sa), including the specific heterodimer subunit of the umami taste (saTAS1R1), three novel sweet taste duplicate genes (saTAS1R2x, saTAS1R2y, saTAS1R2z) and the saTAS1R3 gene common to both umami and sweet taste heterodimers. Gene expression analyses indicate that, although to a different extent, saTAS1R1, saTAS1R2z, saTAS1R2y and saTAS1R3 genes are expressed in oral, gut and brain tissues. Interestingly, the absence of saTAS1R2x mRNA expression in all tissues investigated and the strikingly high levels of saTAS1R2y gene expression in oral and gut tissues indicate that saTAS1R2 paralogs may be under differential evolutionary pressure and may show functional specialization. Furthermore, we identified and cloned two G-protein alpha subunit genes (saG(i)α1 and saG(i)α2) putatively involved in the first steps of intracellular taste signaling in fish. Heterologous transfection studies on multiple saTAS1R heterodimers coupled to saG(i)α1 or saG(i)α2 have just begun to be performed and will contribute to advance important knowledge on fish taste physiology. This study provides an essential groundwork to further elucidate the evolution and functional roles of the TAS1R gene repertoire in seabream, to ultimately develop products enhancing the attractability of fish diets and possibly also regulating digestive/absorptive functions and metabolism through gut sensing mechanisms.