The human bitter taste receptors (TAS2R) are rather well characterised by extensive screenings, resulting in the deorphanisation of 21 of the 25 TAS2Rs. The receptive ranges of receptors of other species, however, initially remained quite unexplored. After realising the importance of comparisons between receptor characteristics of different species in order to fully understand e.g. human TAS2R evolution and function, bitter taste receptor repertoires of other vertebrates were examined. The comprehensive analysis of mouse bitter taste receptors revealed that functional conservation, even among one-to-one orthologues of human and mouse, was rarely seen.
On the contrary, recent research indicated that agonist profiles among related receptors of different species may be more conserved than previously thought. Overlapping agonist profiles were found among distantly related bird species as well as among even more distantly related bony fish species. To see how widespread functional conservation of bitter taste receptors in different species is, we investigated whether the perception of evolutionary stable and hence “old” agonists such as metal salts, that activate the human TAS2R7, is also conserved across species. Thus, we performed functional calcium mobilization assays in transiently transfected HEK293T cells to identify conservation. In doing so, we were able to identify metal ions, especially Mg²⁺, as additional conserved agonists in one-to-one orthologues of the human TAS2R7 of distant relatives of Boreoeutheria, namely desmodus rotundus, cavia porcellus and felis catus.
In conclusion, we can assume that conservation of bitter taste responses between species occurs more often than primarily thought and, in spite of the evolutionary distant relationship of the four investigated species, the conservation of metal ion response of TAS2R7 over millions of years indicates an important physiological function in mammals.