β2-Adrenergic Receptors Chaperone Trapped Bitter Taste Receptor 14 to the Cell Surface as a Heterodimer and Exert Unidirectional Desensitization of Taste Receptor Function

Bitter taste receptors (TAS2Rs) are G-protein-coupled receptors now recognized to be expressed on extraoral cells, including airway smooth muscle (ASM) where they evoke relaxation. TAS2Rs are difficult to express in heterologous systems, with most receptors being trapped intracellularly. We find, however, that co-expression of (2)-adrenergic receptors ((2)AR) in HEK-293T routes TAS2R14 to the cell surface by forming receptor heterodimers. Cell surface TAS2R14 expression was increased by approximate to 5-fold when (2)AR was co-expressed. Heterodimer formation was shown by co-immunoprecipitation with tagged receptors, biomolecular fluorescence complementation, and merged confocal images. The dynamic nature of this interaction was shown by: a gene-dose relationship between transfected (2)AR and TAS2R14 expression, enhanced (up to 3-fold) TAS2R14 agonist stimulation of [Ca2+](i) with (2)AR co-transfection, approximate to 53% decrease in [Ca2+](i) signaling with shRNA knockdown of (2)AR in H292 cells, and approximate to 60% loss of [Ca2+](i) responsiveness in AR knock-out mouse ASM. Once expressed on the surface, we detected unidirectional, conformation-dependent, interaction within the heterodimer, with (2)AR activation rapidly uncoupling TAS2R14 function (approximate to 65% desensitization). Cross-talk was independent of (2)AR internalization and cAMP/PKA, and not accompanied by TAS2R14 internalization. With prolonged -agonist exposure, TAS2R14 internalized, consistent with slow recycling of naked TAS2R14 in the absence of the heterodimeric milieu. In studies of ASM mechanics, rapid cross-talk was confirmed at the physiologic level, where relaxation from TAS2R14 agonist was decreased by approximate to 50% with -agonist co-treatment. Thus the (2)AR acts as a double-edged sword: increasing TAS2R14 cell surface expression, but when activated by -agonist, partially offsetting the expression phenotype by direct receptor:receptor desensitization of TAS2R14 function.