Taste buds respond to more than one taste stimulus because they contain multiple type II cells of different specificities. The type II cells in a taste bud can differ in their expression of taste GPCRs such that each taste bud can respond to multiple taste stimuli. It should be noted that T1R1, T1R2 and T1R3 are often co-expressed in taste bud cells, and, accordingly, responses to both sweet and umami stimuli can be detected in the same cells 41, 114. Most type II cells express one class of taste GPCR - namely, taste receptor type 1 (T1R) or T2R - and correspondingly respond only to one taste quality (for example, sweet or bitter, but not both) 131, 132.
These cells are larger in diameter than type I cells, have sizable spherical nuclei, and function as chemosensory receptors for sugars, amino acids and/or bitter stimuli as they express taste G protein-coupled receptors (GPCRs) and their downstream effectors. Indeed, type I cells may be quite heterogeneous in terms of their gene expression patterns and their functions.Īpproximately one-third of the cells in a taste bud are type II cells. Few other details are known about their function. They express enzymes and transporters that are required to eliminate extracellular neurotransmitters 112, 113, 164 and ion channels that are associated with the redistribution and spatial buffering of K + (REF. Type I cells seem to have glia-like functions. They have narrow, irregularly shaped nuclei, are electron-dense and have wing-like cytoplasmic extensions that ensheath other taste bud cells 162, 163. Type I cells comprise approximately half the total number of cells in a taste bud. One or both of these transporters are hypothesized to be part of an alternative glucose-sensing pathway that is similar to the one used in pancreatic β cells. Glucose transporter type 4 (GLUT4) - which has 12 membrane-spanning segments - transports glucose by facilitative diffusion, whereas sodium/glucose cotransporter 1 (SGLT1) is Na dependent. The epithelial Na channel (ENaC) has three subunits and is thought to transduce salty taste in rodents.
All these taste GPCRs use a common transduction pathway that includes a Gβγ-activated phospholipase C (PLCβ2) and transient receptor potential cation channel subfamily M member 5 (TRPM5). T1R1–T1R3 is an umami receptor, and T1R2–T1R3 is a sweet-taste receptor. T1Rs (sweet-taste and umami receptors) are also GPCRs, but they have long N termini that contain bilobed (venus flytrap) domains and function as dimers that use T1R3 as an obligate subunit. Type 2 taste receptors (T2Rs bitter-taste receptors) are G protein-coupled receptors (GPCRs) that have short amino termini and may function as monomers (not shown) or dimers. Each of these areas has seen many new developments, controversies and clarifications in the past decade. This Review discusses the proteins and pathways that taste buds use to detect stimuli, the communication and modulation that occur between their cells, and the nerve fibres that innervate taste buds, as well as the principles of the coding by which information is conveyed from the periphery to neurons in the CNS. The molecular recognition of tastants, which occurs at the apical tips of taste bud cells, ultimately results in sensory perceptions (for example, sweet, salty, and so on) that guide appetite and trigger physiological processes for absorbing nutrients and adjusting metabolism. The substantial diversity and redundancy of the molecular receptors for these compounds may reflect the importance of identifying nutrients and avoiding chemical threats from the environment. They sample the chemical makeup of foods and beverages for nutrient content, palatability and potential toxicity. Taste buds are the peripheral organs of gustation and are located mainly in the tongue epithelium, although they are also present elsewhere in the oral cavity.