2. Taste sensation
It is the taste cells located in the taste buds of oral epithelium that
are responsible for the perception of taste. There are three types of
taste cells, including glial-like cells (Type
I),
sweet/bitter/umami detectors (Type II), and
sour
receptor cells (Type III) (Figure 1a), which are mainly distributed in
circumvallate (CV) papillae, foliate (FL) papillae, and fungiform (FF)
papillae[34,35]. All three kinds of cells are derived from
K5+/K14+ cells, which form the
precursors, Shh+ basal cells, of all three taste cell
types in low level of β-catenin[34] (Figure 1b) and an average taste
cell lifespan of 8-12 days in mammalians [36] and 10-14 in rodents,
although different types of taste cells showed different
lifespan[37]. Some taste cells transmit taste stimuli through taste
receptors and then generate second messengers while others allow the
stimulus to be transported into the cytoplasm to directly activate
downstream reactions[38]. The most studied is the type II taste
receptors, which means that bitter, sweet, and umami tastes use GPCRs
and second messenger signaling mechanisms to transmit taste signals. The
sour and salty stimulus use ion channels for transduction[39,40].
The information transmission pathway of each taste is different. The
sense of salty taste is mainly completed by the specific sodium salt
taste receptor epithelial sodium channel (epithelial sodium channel,
ENaC) and the non-specific sodium salt taste receptor capsaicin receptor
1 (vanilloid receptor-1 nonselective cation channel 1, TRPV1). It is
recognized on the front of the tongue. Sour taste is the taste sensation
produced by H+ entering the oral cavity to stimulate
the tongue mucosa. Transient receptor potential (TRP) channel members
polycystin kidney dominant 1 like 3 (PKD1L3) and PKD2L1 are participate
in sour sensation at the back of the tongue. The sweet and umami taste
receptors (T1R2/T1R3 and T1R1/T1R3 receptors) distributed in the tongue
epithelium are coupled to the taste-specific G protein α-gustducin
present on the taste buds, releasing Gβγ subunits, and activating
phospholipase C-β2 (phospholipase C-β2), PLC-β2) and adenylyl cyclase
(AC). This will activate the downstream inositol triphosphate (IP3) and
cAMP pathways, and the intracellular Ca2+concentration increases, which in turn activates the transient receptor
potential channel M5 (transient receptor potential channel M5, TRPM5)
depolarizing the cell membrane and releasing neurotransmitters, thereby
producing sweetness. There are differences in the intracellular signal
transduction of sweet receptors at different sites. The bitter taste
receptor proteins (T2Rs) are distributed in the root of the tongue and
combines with the bitter substance dissolved in the liquid phase to
activate the taste cells and depolarize the cell membrane. Then it
causes the nerve cells to post-synaptic excitement and is transmitted to
the bitter taste center of the cerebral cortex[38].