The Umami taste receptor TAS1R1/TAS1R3 is expressed and potentiates contraction of airway smooth muscle
Authors: Sasaki Haruka , Zhang Yi , Emala Charles , Mizuta Kentaro 
Published: May 2023

Introduction: In inflamed tissues, local glutamate concentrations increase due to increased vascular permeability and glutamate release from immune cells. Hence, local concentrations of glutamate in the airways could be increased in asthmatics and may modulate asthmatic symptoms. Classically, glutamate activates both ionotropic and metabolic glutamate receptors. In addition, the umami taste receptor TAS1R1/TAS1R3 which is coupled to Gi and Gs protein also acts as a sensor of glutamate. Although the functional expression of taste receptors outside the oral cavity has been identified, the expression of TAS1R1/TAS1R3 in the airway has never been described. Here, we questioned whether TAS1R1/TAS1R3 is expressed in airway smooth muscle (ASM) and modulates ASM tone. Methods: We evaluated the expression of TAS1R1 and TAS1R3 on human ASM (HASM) cells using RNA-seq, RT-PCR, and immunoblots. The effects of monosodium glutamate (MSG) on cAMP production were evaluated in the presence or absence of Gi protein inhibitor pertussis toxin (PTX; 50 ng/ml), Gs protein inhibitor NF449, TAS1R1/TAS1R3 allosteric modulator (IMP; 150 μM) or the cells were transfected with siRNA targeting TAS1R1/TAS1R3. To evaluate whether the activation of TAS1R1/TAS1R3 modulates acetylcholine (ACh; EC50)-induced ASM contraction, mouse tracheal rings suspended under isometric tension in organ baths were treated with MSG (100 μM) ± IMP, followed by ACh. The contribution of TAS1R1/TAS1R3 to ASM relaxation induced by cumulatively increasing concentration of isoproterenol was also evaluated. Results: We detected the expression of TAS1R1 and TAS1R3 in HASM cells. MSG significantly inhibited forskolin-stimulated cAMP production in HASM cells (84.1 ± 3.0% of forskolin alone; n = 7, P < 0.01), which was potentiated by IMP (72.0 ± 4.9% of forskolin alone; n = 7, P < 0.01) or NF449 (83.8 ± 1.9% of forskolin alone; n = 5, P < 0.01). In contrast, the inhibitory effect of MSG on cAMP production was reversed by pretreatment of the cells with PTX or knockdown of TAS1R1 and TAS1R3. Although MSG alone did not affect basal or ACh-induced contractile ASM tone, MSG potentiated ACh-induced ASM contraction in the presence of IMP (112.4 ± 1.6% of previous ACh-induced ASM contraction; n = 11). Furthermore, cotreatment with MSG and isoproterenol resulted in a leftward shift in the isoproterenol relaxation concentration-response curve compared with treatment with isoproterenol alone (EC50 = 16.3 nM vs. 31.0 nM, respectively), which was attenuated in the presence of IMP (EC50 = 33.6 nM) (n = 6). Conclusion: Functional TAS1R1/TAS1R3 are expressed on ASM, and Gi-signaling is greater than Gs-signaling for cAMP production. Activation of TAS1R1/TAS1R3 potentiates ACh-induced ASM contraction. These findings suggest that activation of the TAS1R1/TAS1R3 and Gi leading to crosstalk potentiation of Gq-coupled receptors could be an important regulator of airway contractile responses. Grant-in-Aid for JSPS Fellows (20J20813) (to Sasaki H.) This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.