Two families of candidate taste receptors in fishes

Abstract

Vertebrates receive tastants, such as sugars, amino acids, and nucleotides, via taste bud cells in epithelial tissues. In mammals, two families of G protein-coupled receptors for tastants are expressed in taste bud cells-T1Rs for sweet tastants and umami tastants (l-amino acids) and T2Rs for bitter tastants. Here, we report two families of candidate taste receptors in fish species, fish T1Rs and T2Rs, which show significant identity to mammalian T1Rs and T2Rs, respectively. Fish T1Rs consist of three types: fish T1R1 and T1R3 that show the highest degrees of identity to mammalian T1R1 and T1R3, respectively, and fish T1R2 that shows almost equivalent identity to both mammalian T1R1 and T1R2. Unlike mammalian T1R2, fish T1R2 consists of two or three members in each species. We also identified two fish T2Rs that show low degrees of identity to mammalian T2Rs. In situ hybridization experiments revealed that fish T1R and T2R genes were expressed specifically in taste bud cells, but not in olfactory receptor cells. Fish T1R1 and T1R2 genes were expressed in different subsets of taste bud cells, and fish T1R3 gene was co-expressed with either fish T1R1 or T1R2 gene as in the case of mammals. There were also a significant number of cells expressing fish T1R2 genes only. Fish T2R genes were expressed in different cells from those expressing fish T1R genes. These results suggest that vertebrates commonly have two kinds of taste signaling pathways that are defined by the types of taste receptors expressed in taste receptor cells.

Identification of RL-TGR, a coreceptor involved in aversive chemical signaling

“…The identity of an endogenous GPCR whose activity is modified by RL-TGR remains unclear, and this merits further investigation. It will be interesting to examine whether RL-TGR is expressed in olfactory or gustatory cells along with GPCRs that are known to function in chemical sensing (3,29). Furthermore, future studies should investigate whether an RL-TGR/GPCR complex underlies the aversive behavioral response in zebrafish, as well as in fish that co-occur with marine sponges containing chemical defense compounds.…”

Characterization of Ligands for Fish Taste Receptors

“…A triple-fluorescence labeling in situ hybridization analysis was performed as described previously (Ishimaru et al, 2005), with the following modifications: adult medaka fish and zebrafish were fixed with 4% paraformaldehyde (PFA) in PBS for 18 h at 4°C, dehydrated with methanol for 24 h at Ϫ20°C, decalcified with 0.5 M EDTA for 18 h at 4°C, cryoprotected with 30% sucrose in PBS, embedded in Tissue-Tek O.C.T. compound (Sakura Finetechnical, Tokyo, Japan) in liquid nitrogen, and sectioned horizontally in 7 m thicknesses.…”

“…Antisense riboprobes for mfT1Rs, zfT1Rs, zfT2Rs, and zfPLC␤2 were synthesized from the clones containing the full coding sequence of the genes, and that for mfPLC␤2 was described previously (Yasuoka et al, 2004;Ishimaru et al, 2005;Aihara et al, 2007). The probes were hydrolyzed to ϳ500 base length before hybridization.…”

“…For taste reception, vertebrates express the two families of G-protein-coupled receptors (GPCRs), T1Rs and T2Rs, in their taste buds (Hoon et al, 1999;Adler et al, 2000;Zhao et al, 2003;Ishimaru et al, 2005). In mammals, the heteromeric T1R1/3 receptor responds to umami tastants such as amino and nucleic acids, whereas T1R2/3 responds to sweet tastants such as sugars, artificial sweeteners, and sweet proteins (Nelson et al, 2001(Nelson et al, , 2002Li et al, 2002;Zhao et al, 2003;Jiang et al, 2004;Nakajima et al, 2006).…”

“…Small fish such as zebrafish (Danio rerio) and medaka fish (Oryzias latipes) are useful model vertebrates because of their short generation span, ease of transgenic manipulation, small genome sizes, and plenty of accumulated sequence data. Bioinformatics of animal genomes has revealed a number of homologous T1R, T2R, and PLC␤2 genes (Yasuoka et al, 2004;Ishimaru et al, 2005;Go, 2006;Shi and Zhang, 2006). We reported previously that parts of taste bud cells in mammals and fish express T1Rs or T2Rs together with PLC␤2 (Asano-Miyoshi et al, 2001;Ishimaru et al, 2005), strongly suggesting that, among vertebrates, there are common mechanisms of taste reception and transduction, although both the ligands for taste receptors in fish and signal transduction pathways in association with receptorligand interactions remain to be elucidated.…”