About this Research Topic
Taste is one of the five “classic” senses, and initiates in taste receptor cells in the oral cavity of humans and other mammals. Yet, the same taste signaling cascades present in taste receptor cells are also widely expressed throughout the body, and they are involved in a wide variety of functions in tissues and organs outside the mouth. In the gastrointestinal tract sweet taste receptors (i.e., T1R2+T1R3 heterodimer) contribute to glucose sensing and energy balancing, umami and other amino acids receptors reflect on protein-related nutrients (i.e. T1R1+T1R3, CaSR, GPRC6A, GPR92), and bitter taste receptors (T2Rs) mediate satiety hormone secretion, and innate immune responses to parasite infection. In the respiratory system, T2R activation increases airway ciliary beat frequency, but paradoxically relaxes airway smooth muscle. In the genitourinary system, T2Rs participate in spermatogenesis and mediate a reflex loop in the urethra, leading to bladder contraction. With their ubiquitous expression in the body, taste sensing systems may underlie many unknown biological functions in the extra-oral organs. Intriguingly, the expression of TAS1Rs and TAS2Rs is altered under several pathological conditions, and their polymorphisms have been linked to several human disorders. However, the underlying mechanisms for these alterations are unclear and as is whether the polymorphisms cause human diseases. Thus this Research Topic welcomes research articles and review papers that will reveal or deepen the understanding of the functions mediated by TAS1Rs and TAS2Rs in the extra-oral organs, and the roles of these receptor families in different diseases. Of particular interest is research using molecular and genetic approaches to answer these questions.
In addition, fat or fatty acids have been recognized as the triggers of a potential sixth taste in animals and humans. Similar to the T1Rs and T2R families, the receptors for fatty acids, e.g., CD36, GPR40 (FFAR1), GPR41 (FFAR3), GPR43 (FFAR2), GPR84 and GPR120 (FFAR4), are also expressed in a number of cell types throughout the body. In the gastrointestinal tract and hypothalamic nucleus, fatty acids act on these receptors to critically regulate energy balance by changing ingestive behavior, energy storage and utilization.
Significantly, the sensitivity of these receptors to fatty acids in obese individuals is lower than that in lean individuals. This may account for excess fat consumption in obese individuals and contribute to other diseases such as type 2 diabetes. Fatty acid receptors are present in pancreas, immune cells and other extra-oral organs, but the biological significance of these receptors in these tissues is yet to be fully explored. This Research Topic also welcomes research articles and review papers which address emerging roles mediated by this class of receptors and the molecular mechanisms underlying these roles in the extra-oral organs.
Historically, taste is considered to be responsible for differentiating dietary components in food, and to have been evolved under food selective pressure. With the increasing number of biological functions being mediated by extra-oral taste receptors, it raises a possibility whether the evolution of taste receptors may also be attributed to the selective pressure related to their biological functions in the extra-oral organs. Hence this Research Topic further welcomes review papers, opinions and research articles that address the evolution of taste receptors in the extra-oral systems.
Keywords: Chemosense, Bitter taste, Sweet taste, Nutrient sensing, G-protein-coupled receptors
Important Note: All contributions to this Research Topic must be within the scope of the section and journal to which they are submitted, as defined in their mission statements. Frontiers reserves the right to guide an out-of-scope manuscript to a more suitable section or journal at any stage of peer review.