Insights into the Function and Evolution of Taste 1 Receptor Gene Family in the Carnivore Fish Gilthead Seabream (Sparus aurata)

Abstract

A plethora of molecular and functional studies in tetrapods has led to the discovery of multiple taste 1 receptor (T1R) genes encoding G-protein coupled receptors (GPCRs) responsible for sweet (T1R2 + T1R3) and umami (T1R1 + T1R3) taste. In fish, the T1R gene family repertoires greatly expanded because of several T1R2 gene duplications, and recent studies have shown T1R2 functional divergence from canonical mammalian sweet taste perceptions, putatively as an adaptive mechanism to develop distinct feeding strategies in highly diverse aquatic habitats. We addressed this question in the carnivore fish gilthead seabream (Sparus aurata), a model species of aquaculture interest, and found that the saT1R gene repertoire consists of eight members including saT1R1, saT1R3 and six saT1R2a-f gene duplicates, adding further evidence to the evolutionary complexity of fishT1Rs families. To analyze saT1R taste functions, we first developed a stable gene reporter system based on Ca2+-dependent calcineurin/NFAT signaling to examine specifically in vitro the responses of a subset of saT1R heterodimers to L-amino acids (L-AAs) and sweet ligands. We show that although differentially tuned in sensitivity and magnitude of responses, saT1R1/R3, saT1R2a/R3 and saT1R2b/R3 may equally serve to transduce amino acid taste sensations. Furthermore, we present preliminary information on the potential involvement of the Gi protein alpha subunits saGαi1 and saGαi2 in taste signal transduction.

Exploring the potential for an evolutionarily conserved role of the taste 1 receptor gene family in gut sensing mechanisms of fish

“…The highest level of expression and abrupt rise of sa T1R2b expression following first exogenous feeding and its earlier appearance in oropharyngeal tissues as deduced by the whole mount in situ analyses, suggests that this paralog may be playing major roles related to feeding. In support of this hypothesis, sa T1R2b/R3 was the most responsive and sensitive heterodimer to L-amino acid stimulations in this species ( Angotzi et al., 2020 ). Paradoxically, the sa T1R3 gene encoding the shared subunit of receptors signaling both sugar (T1R2/T1R3) and protein (T1R1/T1R3) rich foods in mammals ( Roper, 1989 ; Hoon et al., 1999 ; Adler et al., 2000 ; Yarmolinsky et al., 2009 ), and responding to a wide spectrum of L-amino acids (T1R1/T1R3 and T1R2 n /T1R3) in fish ( Oike et al., 2007 ), was among the lowest expressed gene throughout the larval stages analyzed.…”

“…In a recent study, we reported the complete T1R gene repertoire of gilthead seabream, consisting of eight members including sa T1R1 , sa T1R3 and six sa T1R2 ( a-f ), and functionally characterized the in vitro responses of a subset of heterodimers, namely sa T1R1 / R3 , sa T1R2a / R3 and sa T1R2b / R3 , to L-amino acids and sweet ligands ( Angotzi et al., 2020 ). Here, we further explored the mRNA expression profiles of all sa T1R in both larval and adult tissues.…”

“…Our findings are in accordance with previous studies that reported several taste receptors and taste signaling components in the GIT of different fish species ( Polakof and Soengas, 2013 ; Latorre et al., 2013 ; Ronnestad et al., 2016 ; Yuan et al., 2020 ; Calo et al., 2021 ; Kinnamon, 2012 ; Morais, 2017 ), reinforcing the hypothesis that the T1R-mediated gut sensing mechanisms could be conserved throughout vertebrate evolution. The significantly higher levels of sa T1R3 expression observed in oropharyngeal and GIT tissues suggest that this gene might be locally demanded at higher transcriptional rates due to heterodimerization with sa T1R1 or sa T1R2 subunits, since sa T1R3 / sa T1R3 homodimers do not seem to respond to L-amino acids stimulations in this species ( Angotzi et al., 2020 ). However, the existence of T1R3 homo-oligomers cannot be dismissed since these have been described in mammals and proposed to sense calcium and magnesium taste ( Nelson et al., 2001 ).…”

“…On the other hand, the Gα-gust system is absent in the genome of both amphibians and teleost fishes as a result of 2 independent gene losses in their last common ancestors ( Oka and Korsching, 2011 ; Ohmoto et al., 2011 ), somehow implying that other G(i)α-related proteins might be involved in the initial steps of taste signaling in these lineages. In line with this hypothesis, in a recent study where we comprehensively described the T1R gene repertoire of the carnivorous marine fish gilthead seabream ( Sparus aurata [ sa ]), and it was also shown in vitro that heterologous expression of sa T1R heterodimers co-transfected with the Gi alpha protein subunits saG(i)α1 and saG(i)α2 triggered both stimulatory and inhibitory taste transduction mechanisms upon amino acid activation ( Angotzi et al., 2020 ). Hence, the overall emerging picture suggests a large degree of conservation of the T1R-mediated taste signaling across vertebrates, including fish.…”

“…From an evolutionary perspective, the 3 T1R orthologs are conserved across vertebrates, including fish, whose T1R families have greatly expanded mostly due to additional T1R2 duplicates that apparently evolved to increase taste plasticity for amino acid sensing ( Hashiguchi et al., 2007 ; Baldwin and Ko, 2020 ; Oike et al., 2007 ; Angotzi et al., 2020 ). Emerging evidences based on quantitative molecular studies indicate that several taste receptors and canonical components of T1R transduction signaling are also present in the fish GIT, suggesting that the T1R-mediated gut sensing mechanisms could have been conserved during evolution ( Polakof and Soengas, 2013 ; Latorre et al., 2013 ; Ronnestad et al., 2016 ; Calo et al., 2021 ).…”