Evolution of umami taste receptor (T1R1-T1R3) system in vertebrates and its role in gut-brain axis communication

The ability to taste umami and sweetness depends on receptors that can detect the taste molecules, so-called taste receptors type 1 (T1R). In mammals, the umami taste receptor (T1R1-T1R3) is involved in sensing amino acids, while the sweet taste receptor (T1R2-T1R3) is involved in sensing carbohydrates and artificial sweeteners. These receptors are not only present in the oral cavity but also in the gastrointestinal tract, located in specialized cells named enteroendocrine cells. These cells face the gut lumen and will release hormones as a response to receptor activation. These receptors are thus involved in the control of several digestive functions, including intestinal transit, release of digestive enzymes, nutrient transport, and stimulate pathways to the brain that control appetite and food intake. In contrast to mammals, both T1R1-T1R3 and T1R2-T1R3 may equally sense amino acids in teleost species. This supports previous theories that the taste receptors have evolutionary adapted in close relationship with the species diet. In this context, GUTASTE has explored all taste receptors (t1r1, t1r2 and t1r3 genes) and other nutrient sensors. The additional salmonid-specific whole genome duplication has resulted in a higher number of t1r genes in Atlantic salmon when compared to for example zebrafish. This has open up for new functions or division of functions for the different paralogues. Following this, our data show that salmon t1r1 like 2 gene is highly expressed in the oropharyngeal area but is not detected in the pyloric caeca, midgut or hindgut, in contrast to the paralog t1r1 like 1, which has a widespread distribution and is also found in the gastrointestinal tract. This indicates that in Atlantic salmon taste receptors are differently distributed among the tissues and therefore may exert different functions. Interestingly, both in Atlantic salmon and zebrafish the mRNA expression of t1r1 and t1r2 seem to be modulated by the presence of nutrients while t1r3 is not. These results support that the taste receptors are present in the gut of teleost fishes and they do play a role in nutrient sensing. To explore the nutrient receptors response-specificity to L-amino acids (and other ligands), we have generated receptor specific cell lines and established assays that measure the activated intracellular pathways. The different methods were initially established using the calcium sensing receptor and is now established for different salmon and zebrafish taste receptors. This tool may be further used for screening nutrients and feed ingredients for fish feeds. Furthermore, we also have generated three zebrafish t1r1 mutant lines using the CRISPR-Cas9 method that yielded a truncated protein. After further validation steps, they will become available in our lab as an important tool in education and research to investigate the physiological consequences of disrupting the umami taste receptor and how this affects the appetite response to amino acids and growth. The theoretical background of the project and results have been presented at the Summer School Fish Physiology and Sustainable Aquaculture (Japan in 2018 and Bergen in 2019). Results from an experiment on appetite control conducted during the 2019 course has resulted in a research note publication. Additionally, results about the nutrient sensing receptors have been presented at the national and international conferences. GUTASTE results will continue to produce results and several manuscripts have been submitted or are under preparation.

The GUTASTE project expanded our knowledge on the function of taste receptors, and other nutrient sensing receptors, in the gut of vertebrates, particularly teleost fishes. Specifically, we generated novel insights about the location of these receptors in the gut of fish species, and their response to feeding in general and specific diets. Furthermore, the project has produced a novel in-vitro method to screen for amino acids and feed ingredients. This method will allow to significantly reduce the amounts of fish used in feeding trials. The generation of a zebrafish t1r1 knockout line provides a tool for education and research, enabling us to explore further the functional importance of the umami taste receptor in teleosts. GUTASTE was a steppingstone for the PL establishment as a young female researcher at UiB and to strengthen her national and international network. The project also resulted in follow-up projects that partly build on the achieved output and developed techniques.

Gut taste and gut feeling are not just catch phrases! The gastrointestinal (GI) tract monitors its content continuously in the same way as the tongue, using similar G-protein-coupled taste receptors and transduction mechanisms that are responsible for oral umami, sweet and bitter taste perception. These receptors "taste" (sense) the luminal content and activate neuronal and hormonal signalling pathways that mediate protective functions of the epithelial barrier, changes in gastric emptying and intestinal transit, release of digestive enzymes, nutrient transport, and also affect the control of food intake (hunger and satiety) and metabolism. The umami taste (T1R1-T1R3) receptor recognizes amino acids, which are indispensable nutrients for animals and essential for a range of metabolic pathways and survival. Gut sensing is a new and emerging field that has been poorly explored in mammals and completely disregarded in non-mammalian species, resulting in numerous important knowledge gaps. A better understanding of how the gut senses amino acids through receptors like T1R1-T1R3 will provide biomedical insights and help to define novel drug targets and feed ingredients. GUTASTE wants to explore the vertebrate umami taste receptor system and understand its role in amino acid sensing and subsequent physiological functions. Specifically, this project will: 1) determine the role of vertebrate's umami taste receptor in amino acid sensing and ligand preference specialization; 2) characterize physiological functions of l-amino acid T1R1-T1R3 sensing in the gut, such as peristaltic reflex and hormone/peptide release, and in the gut-brain axis; 3) provide a method to study the physiological consequences of T1R1-T1R3 receptor disruption on gut function and food intake control. To address these research questions, GUTASTE will make use of breakthrough techniques and international collaboration with a world-leading specialist in gut biomedicine and gut-brain interactions.