Identification and characterization of novel ligands for human bitter taste receptor T2R7
Humans have five basic taste sensations which are bitter, sweet, umami, salt and sour. Salt and sour are mediated by ion channel-linked receptors, while bitter, sweet and umami are sensed by cell surface G protein-coupled receptors (GPCRs). Among the five basic taste modalities, the bitter taste signaling mechanism is the most complex and least understood. This is in part due to the large number of bitter taste receptors (T2Rs) in humans (n=25), and the wide range of bitter compounds they detect. The T2Rs are expressed in the oral cavity as well as in many extraoral tissues. The activation of T2Rs in the oral cavity elicits bitter taste sensation, while the activation of the extraoral T2Rs causes various physiological and pathophysiological responses. Recent findings showed that T2R activation caused muscle relaxation and bronchodilation of pre-contracted airway smooth muscle. Moreover, bacterial quorum sensing molecules were shown to activate T2Rs in extraoral tissues. In view of the importance of T2R functions in extraoral tissues, it is of great value to study the structure-function relationship of T2Rs and identify efficient ligands. Hitherto, there have been few studies published involving human bitter taste receptor 7 (T2R7). Only few agonists have been identified for the T2R7 receptor, and no structure-function studies on the T2R7 receptor have been reported. In this study, different compounds including known T2R7 agonists, common bitter compounds, antibiotics and quorum sensing molecules were tested for their ability to activate or inhibit T2R7 heterologously expressed in HEK293T cells. Results suggest that T2R7 is activated by novel ligands including dextromethorphan (DXM), diphenhydramine (DPH), thiamine, tobramycin and erythromycin. In contrast, beef protein hydrolysates purified by RP-HPLC quenched quinine activated T2R7 signal. This suggests that beef hydrolysates may contain potent T2R7 blockers. To understand the structure of the ligand binding pocket in T2R7, molecular model guided site-directed mutagenesis was pursued. Eighteen mutants were made at nine amino acid positions in the ligand binding pocket of T2R7 and their expression was characterized by flow cytometry. All of the mutants were properly expressed on the cell surface. Functional characterization of the mutants including intracellular calcium mobilization in response to quinine and DXM treatment allowed mapping of the ligand binding pocket of T2R7. Taken together, this study identified novel ligands for T2R7 and elucidated the important amino acids involved in T2R7 ligand binding.