. The long-term goal of the project is to understand the mechanisms that underlie the regulation of gut motility by nutrient receptors and offer insights into the development of potential new therapeutic targets for motility disorders. Recent studies have shown the presence of nutrient receptors outside the oral cavity, especially in the gut. Of the 5 basic tastes, umami, sweet and bitter are sensed via G protein-coupled receptors that include heterodimers of T1Rs and T2Rs. The heterodimeric umami receptor (T1R1/T1R3) is sensitive to L- amino acids such as monosodium glutamate, the heterodimeric T1R12/T1R3 is sensitive to sugars, and the heterodimers of T2Rs are sensitive to bitter compounds. Functional bitter taste receptors have been demonstrated in smooth muscle cells of human and mouse gut and activation of these receptors by different bitter compounds induced contraction and relaxation in an agonist- and region-specific manner. However, T1R expression and function have not been described in smooth muscle. Our preliminary results showed expression of the T1Rs and the G protein, ?gustducin (G?gust), in gastric smooth muscle, as well as decreased contraction in gastric muscle strips in response to activation of T1R1/T1R3 and T1R2/T1R3. Canonical signaling of T1R and T2R receptors activates G?gust, to increase [Ca2+]i via PLC-?2 and depolarization of the plasma membrane, which is facilitated by TRPM5. However, our preliminary data show T1R1/T1R3 activation decreases [Ca2+]i in gastric smooth muscle, indicating an alternative cascade. Based on our results, and published studies, we hypothesize that T1R1/T1R3 (umami) and T1R2/T1R3 (sweet), expressed on GI smooth muscle, alters GI motility through G proteins (G?gust/G?i/G?s) coupled to signaling pathway that lead to a decrease in [Ca2+]i. In this proposal, the studies will elucidate the stomach region-specific expression of T1Rs and their signaling components (Aim 1), the molecular mechanism that leads to relaxation in response to sweet and umami receptor ligands (Aim 2), and region-specific motility patterns induced by these ligands to promote relaxation (Aim 3) will be measured by the state-of-the art technique of spatiotemporal mapping. As the GI tract acts as a gateway for the effect of nutrients, understanding the nutrient receptor function opens the possibility that GI functions can be modulated by targeting these receptors. Advances in knowledge will also guide the development of new targets to treat motility disorders. provide necessary training to Dr. Crowe in several techniques ranging from cell biology to functional studies in the field of neurogastromotility. At the conclusion of this study, we will have gained important knowledge about the regulation of gut motility by nutrient receptors. Also, Dr. Crowe will be well poised to submit a K01 award by extending the studies to evaluate the nutrient receptors in disease models. Considering nutrient receptors are instrumental in insulin regulation, evaluating the mechanisms and function of these receptors in the gastrointestinal system in metabolic disorder models, such as diabetes, would give insight into normal and abnormal gut physiology and potential new therapeutic targets.
. Recent studies have shown the presence of nutrient receptors outside the oral cavity, especially in the gut, where these receptors are expressed not only in the enteroendocrine cells, but also in other cell types including smooth muscle cells. The majority of nutrient receptor research in the gut has focused on the mucosa and the functional consequence of activation of these receptors in smooth muscle is unclear. Therefore, the objective of this proposal is to understand the mechanisms that underlie the regulation of gut motility by the umami and sweet nutrient receptors in gastric smooth muscle. Understanding the nutrient receptor function opens the possibility that GI functions can be modulated by targeting these receptors.
