Functional expression of the proton channel OTOP1 in brown adipocytes

Otopetrin 1 (OTOP1) is a proton (H⁺) channel that mediates perception of acidic taste in type III taste receptor cells. According to data from an mRNA expression database, Otop1 shows high expression in mouse brown adipose tissue (BAT). BAT is a thermogenic organ composed of brown adipocytes (BAs), which play a pivotal role in non-shivering thermogenesis. A study using Otop1 knockout (KO) mice suggested OTOP1 could influence BAT function in certain conditions. However, critically, it has not been investigated whether OTOP1 is functionally expressed in BAs. Demonstrating that OTOP1 is functionally active in BAs is essential for interpreting the Otop1 KO results and for elucidating its role in BAs. Therefore, in the present study, we examined the functional expression of OTOP1 in BAs using Otop1 KO mice. Whole-cell patch clamp recordings revealed that differentiated BAs exhibited Otop1-dependent H⁺ currents. Cytosolic pH measurements revealed Otop1-dependent pH reduction in response to applications of acidic solutions. These results indicate that OTOP1 is active on the plasma membrane of BAs. Therefore, the present findings allow future studies of OTOP1's function in BAs to proceed under the assumption of its endogenous activity. Moreover, BAs can serve as a useful cellular model for investigating the function of endogenously expressed OTOP1, alongside type III taste receptor cells.