Epitope tagging with genome editing in mice reveals that the proton channel OTOP1 is apically localized and not restricted to Type III “sour” taste receptor cells

The gustatory system allows animals to assess the nutritive value and safety of foods prior to ingestion. The first step in gustation is the interaction of taste stimuli with one or more specific sensory receptors, that are generally believed to be present on the apical surface of the taste receptor cells. However, this assertion is rarely tested. We recently identified OTOP1 as a proton channel and showed that it is required for taste response to acids (sour) and ammonium. Here we examined the cellular and subcellular localization of OTOP1 by tagging the endogenous OTOP1 protein with an N-terminal HA epitope (HA-OTOP1). Using both male and female HA-OTOP1 mice and high-resolution imaging, we show that OTOP1 is strictly localized to the apical tips of taste cells throughout the tongue and oral cavity. Interestingly, immunoreactivity is observed in the actin-rich taste pore above the tight junctions defined by Zonula Occludens-1 (ZO-1) and also immediately below these junctions. Surprisingly, OTOP1 immunoreactivity is not restricted to Type III taste receptor cells (TRCs) that mediate sour taste but is also observed in glia-like Type I TRCs proposed to perform housekeeping functions, a result that is corroborated by scRNA-seq data. The apical localization of OTOP1 supports the contention that OTOP1 functions as a taste receptor and suggests that OTOP1 may be accessible to orally available compounds that could act as taste modifiers. Significance Statement It is generally accepted that humans and other vertebrates can detect five basic tastes, each mediated by a unique receptor. Recently the receptor for sour taste was identified as the proton channel OTOP1. Here we show that OTOP1 is expressed at the apical surface of taste receptors cells, consistent with a sensory function. Surprisingly, OTOP1 is not restricted to Type III taste cells that detect sour tastes but is also expressed by glia-like taste cells, where it may play a role in removing excess protons. These results provide insight and tools applicable to understanding the contribution of OTOP1 to cell physiology and pathology in other contexts where the channel is expressed such as in the vestibular system.