New Discovery Of How Sour Taste Links To Sense Of Balance

An entirely new class of ion channels had been discovered by scientists at the University of Southern California. There channels let protons into cells and are important to inner ear balance, and can be found in taste cells at the back of the tongue that respond to sour flavors, as published in Science.

Whether a solution is basic or acidic is controlled by protons that set pH levels. Protons don’t cross cell membranes, they get transported across by proteins like ion channels. Gene encoding an ion channel will let protons leaving cells has been identified, but it is not known if one or several genes are required to form an ion channel that lets the protein into cells. Investigations into sour taste has identified the otopetrin gene family as encoding proton conducting ion channels.

Otopetrin gene family has been identified as playing important roles in balance. It has been observed that mice with mutations in Otop1 are not able to right themselves, and they are called tilted or tlt. Encoded protein functions and why mutations in the gene cause a vestibular defect are unknown. While investigating taste perception researchers discovered that Otop1 encodes a proton channel suggesting hints as to how Otop1 serves to contribute to the inner ear function of balance.

Sour taste is the perception of present acidic substances, which will typically have high proton concentration. Sour taste cells have an ion channel that will respond to or transport protons. Biophysical approaches were used to show how protons enter taste cells through specialized proton channels in the cell membranes, but the structural properties and gene encoding of the proton channels were unknown. Using a molecular genetics technique called RNAseq to identify which genes were expressed by sour taste cells specifically and not other types of taste cells genes was used to conduct tests on candidates one by one until the one was found to be producing a proton conducting protein when it was introduced into cells that did not have any proton conducting channels, a process which took years.

Two other related genes in vertebrates Otop2 and Otop3, the gene family is represented in the Drosophila melanogaster fruit fly. Otopetrins are structurally different than all other ion channels. All of the otopetrins form proton channels suggesting that they are evolutionarily conserved. Each otopetrin has a distinct distribution in a variety of tissues which includes the nerves, eye, ear, digestive tract, tongue, and reproductive organs.

In the taste system totpetrins may be involved in the sensing of acids as a part of the sour taste perception, functions of these proton channels in other tissues is still unknown. In the vestibular system, otopetrin is required for the function and formation of otoconia, which are calcium carbonate crystal structures that sense acceleration and gravity. Researchers suggest that the otopetrins maintain pH appropriate formation of otoconia with the defect in the tilt mice being due to a dysregulation of pH.