Oike Hideaki, Nagai Toshitada, Furuyama Akira, Okada Shinji, Aihara Yoshiko, Ishimaru Yoshiro, Marui Takayuki, Matsumoto Ichiro, Misaka Takumi, Abe Keiko
Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo-ku, Tokyo 113-8657, Japan.
J Neurosci. 2007 May 23;27(21):5584-92. doi: 10.1523/JNEUROSCI.0651-07.2007.
Recent progress in the molecular biology of taste reception has revealed that in mammals, the heteromeric receptors T1R1/3 and T1R2/3 respond to amino acids and sweeteners, respectively, whereas T2Rs are receptors for bitter tastants. Similar taste receptors have also been characterized in fish, but their ligands have not been identified yet. In the present study, we conducted a series of experiments to identify the fish taste receptor ligands. Facial nerve recordings in zebrafish (Danio rerio) demonstrated that the fish perceived amino acids and even denatonium, which is a representative of aversive bitter compounds for mammals and Drosophila. Calcium imaging analysis of T1Rs in zebrafish and medaka fish (Oryzias latipes) using an HEK293T heterologous expression system revealed that both T1R1/3 and a series of T1R2/3 responded to amino acids but not to sugars. A triple-labeling, in situ hybridization analysis demonstrated that cells expressing T1R1/3 and T1R2/3s exist in PLCbeta2-expressing taste bud cells of medaka fish. Functional analysis using T2Rs showed that zfT2R5 and mfT2R1 responded to denatonium. Behavior observations confirmed that zebrafish prefer amino acids and avoid denatonium. These results suggest that, although there may be some fish-specific way of discriminating ligands, vertebrates could have a conserved gustatory mechanism by which T1Rs and T2Rs respond to attractive and aversive tastants, respectively.
味觉受体分子生物学的最新进展表明，在哺乳动物中，异源三聚体受体T1R1/3和T1R2/3分别对氨基酸和甜味剂作出反应，而T2Rs是苦味剂的受体。在鱼类中也鉴定出了类似的味觉受体，但其配体尚未确定。在本研究中，我们进行了一系列实验以鉴定鱼类味觉受体的配体。斑马鱼（Danio rerio）的面神经记录表明，鱼类能够感知氨基酸，甚至是苯甲地那铵，苯甲地那铵是哺乳动物和果蝇厌恶苦味化合物的代表。使用HEK293T异源表达系统对斑马鱼和青鳉（Oryzias latipes）中的T1Rs进行钙成像分析，结果显示T1R1/3和一系列T1R2/3均对氨基酸有反应，但对糖类无反应。三重标记原位杂交分析表明，表达T1R1/3和T1R2/3s的细胞存在于青鳉表达PLCβ2的味蕾细胞中。使用T2Rs进行的功能分析表明，zfT2R5和mfT2R1对苯甲地那铵有反应。行为观察证实，斑马鱼偏好氨基酸并避开苯甲地那铵。这些结果表明，尽管可能存在一些鱼类特有的区分配体的方式，但脊椎动物可能具有保守的味觉机制，即T1Rs和T2Rs分别对吸引性和厌恶性味觉物质作出反应。
