Figure 12. Working model illustrating a possible functional role of taste receptor signaling in taste cells and spermatozoa.

[A] Model for the transduction cascade of the umami receptor in taste cells. On the left, a schematic drawing of the onion-like structure of a single taste bud formed by elongated taste cells is shown. The peripheral ends of the 50–100 taste cells in one taste bud terminate at the gustatory pore; taste information is coded by afferent nerve fibers which innervate the taste buds and come close to type II receptor cells but only form conventional chemical synapses with the basolateral membrane of type III taste cells. In taste cells, the Tas1r1 and Tas1r3 receptors form a functional dimer which is able to recognize amino acids such as MSG. Upon ligand binding, the umami receptor activates a trimeric G Protein consisting of α-gustducin [αGus] and β₃ and γ₁₃ [βγ]. The βγ subunit activates phopholipase Cβ₂ [PLC] which cleaves phosphatidylinositol 4, 5-bisphosphate [PIP₂] to inositol trisphoshate [IP₃] and diacylglycerol [DAG]. IP₃ mediates an increase in intracellular calcium by activation of calcium channels in the endoplasmic reticulum [ER] and subsequently an influx of calcium through ion channels in the plasma membrane [TRPM5]. Simultaneously, released α-gustducin can activate phosphodiesterase, resulting in a decrease of intracellular levels of cyclic adenosine monophosphate [cAMP]. A crosstalk between the two pathways exists through a cAMP regulated activation of protein kinas A [PKA] which inhibits PLC and the IP₃-receptor in the ER. This mechanism may ensure adequate Ca²⁺ signaling to taste stimuli by keeping the taste cell in a tonically suppressed state. The drawing was modified from Ref. [45] and [109]. [B] Putative model of Tas1 taste receptor signaling in spermatozoa. The schematic drawing in the left signifies the sperm's journey in the different sections of the female genital tract [uterus, oviduct, ampulla] which sperm have to transit to reach the egg in the ampullar region of the oviduct (dotted red line). In sperm cells, the Tas1r1 protein [Tas1r1] may dimerize with its taste partner Tas1r3 or with a yet not identified receptor [R?]. G protein activation results in the release of a G protein α-subunit [Gα] which activates phosphodiesterase [PDE], thus leading to the hydrolysis of cAMP. In this model, an activation of the receptor dimer [Tas1r1/R?] by chemosensory ligands within the different regions of the female genital tract (red rhoms) or a constitutively active receptor may ensure low cAMP levels, thereby preventing cAMP-triggered maturation processes of the sperm, like capacitation, motility or acrosome reaction, before the sperm reaches the egg in the ampullary part of the oviduct. If the simultaneously released Gβγ complex [βγ] indeed stimulates PLC in analogy to taste cells or alternatively activates potassium [K⁺] channels in sperm, is currently not clear. Constant cAMP hydrolysis can be overcome during sperm maturation either by an decrease in taste receptor activation controlled by changes in the composition of chemical components in the different fluids of the female genital tract or by an increase in [Ca²⁺]_i, or high bicarbonate concentration which would lead to an activation of the soluble adenylatecyclase [sAC] in spermatozoa. For seek of simplicity, regulatory effects of PKA activation or EPAC stimulation on calcium channels or the IP₃ receptor are omitted in the model.