Functional diversification of taste cells in vertebrates

Abstract

Tastes are senses resulting from the activation of taste cells distributed in oral epithelia. Sweet, umami, bitter, sour, and salty tastes are called the five "basic" tastes, but why five, and why these five? In this review, we dissect the peripheral gustatory system in vertebrates from molecular and cellular perspectives. Recent behavioral and molecular genetic studies have revealed the nature of functional taste receptors and cells and show that different taste qualities are accounted for by the activation of different subsets of taste cells. Based on this concept, the diversity of basic tastes should be defined by the diversity of taste cells in taste buds, which varies among species.

Figure 1. Diverse array of taste cells in mouse and zebrafish

Types of taste cells are illustrated with specific molecular features such as taste receptors and (in)dispensable markers. Their ligands identified thus far are indicated. Unidentified receptors and ligands are shown by question marks. The unidentified receptors in PLC-β2/TRPM5-expressing taste cells are presumably GPCRs [32]. Pkd2l1 is not a sour receptor channel, because its knockout had little effect to sour response in gustatory neurons [70]. ENaC is indispensable for NaCl attraction [48], but it remains unclear whether it is the specific receptor. Many types of T1R(s)-expressing and T1R(s)/T2R(s)-expressing cells exist in zebrafish, depending on the expression pattern of receptor genes. Taste buds in zebrafish have many cells that do not express PLC-β2 or TRPM5, although little is known for their molecular features.