Lactisole Interacts with the Transmembrane Domains of Human T1R3 to Inhibit Sweet Taste

Abstract

The detection of sweet-tasting compounds is mediated in large part by a heterodimeric receptor comprised of T1R2؉T1R3. Lactisole, a broad-acting sweet antagonist, suppresses the sweet taste of sugars, protein sweeteners, and artificial sweeteners. Lactisole's inhibitory effect is specific to humans and other primates; lactisole does not affect responses to sweet compounds in rodents. By heterologously expressing interspecies combinations of T1R2؉T1R3, we have determined that the target for lactisole's action is human T1R3. From studies with mouse/ human chimeras of T1R3, we determined that the molecular basis for sensitivity to lactisole depends on only a few residues within the transmembrane region of human T1R3. Alanine substitution of residues in the transmembrane region of human T1R3 revealed 4 key residues required for sensitivity to lactisole. In our model of T1R3's seven transmembrane helices, lactisole is predicted to dock to a binding pocket within the transmembrane region that includes these 4 key residues.Taste is a primal sense that enables diverse organisms to identify and ingest sweet-tasting nutritious foods and to reject bitter-tasting environmental poisons (1). Taste perception can be categorized into five distinct qualities: salty, sour, bitter, umami (amino acid taste), and sweet (1). Salty and sour depend on the actions of ion channels. Bitter, umami, and sweet depend on G-protein-coupled receptors (GPCRs) 1 and coupled signaling pathways. Sweet taste in large part depends on a heterodimeric receptor comprised of T1R2ϩT1R3 (2-5).The T1R taste receptors (T1R1, T1R2, and T1R3) are most closely related to metabotropic glutamate receptors (mGluRs), Ca 2ϩ -sensing receptors (CaSRs), and some pheromone receptors (6 -10). All of these receptors are class-C GPCRs, with the large clam shell-shaped extracellular amino-terminal domain (ATD) characteristic of this family. Following the ATD is a cysteine-rich region that connects the ATD to the heptahelical transmembrane domain (TMD); following the TMD is a short intracellular carboxyl-terminal tail. The solved crystal structure of the ATD of mGluR1 identifies a "Venus flytrap module" (VFTM) involved in ligand binding (11). The canonical agonist glutamate binds within the VFTM in a cleft formed by the two lobes of this module to stabilize a closed active conformation of the mGluR1 ATD. In contrast, several positive and negative allosteric modulators of class-C GPCRs have been identified and shown to act via binding not within the VFTM but instead within the TMD (12-16).Over the past few decades, multiple models of the sweet receptor's hypothetical ligand binding site have been generated based on the structures of existing sweeteners but without direct knowledge of the nature of the sweet receptor itself. A consensus feature of these models is the presence of A-H-B groups, in which the AH group is a hydrogen donor and the B group is an electronegative center. These models have explanatory and predictive value for some, but not all sweeteners, suggesting th...

Lipid-Lowering Pharmaceutical Clofibrate Inhibits Human Sweet Taste

“…It has been shown that the T1R3 inhibitor sodium lactisole increases plasma glucose 'area under the curve' (AUC) response to a glucose load (Gerspach et al 2011). This effect is consistent with reduced stimulation of insulin by T1Rs (Jiang et al 2005;Hamano et al 2015). Whether other sweet taste inhibitors have similar physiological effects on human physiology is unclear.…”

“…T1R2-T1R3 is activated by several mono-and disaccharides as well as high potency sweeteners (HPSs) (Cui et al 2006). In humans it is inhibited by sodium lactisole, an inverse agonist which binds the transmembrane domain of human-T1R3 (Jiang et al 2005;. A growing body of evidence shows that T1R2-T1R3 is expressed in tissues throughout the body and that it serves physiological roles in glucose metabolism, insulin secretion, lipid metabolism, adipocyte function, and reproductive health Mosinger et al 2013;Smith et al 2016).…”

An Automated System for the Analysis of G Protein-Coupled Receptor Transmembrane Binding Pockets: Alignment, Receptor-Based Pharmacophores, and Their Application

“…We have also shown that the developed method is not limited to class A GPCR receptors, where existing patterns of highly conserved residues in the transmembrane region of the GPCRs help to guide the 7TM alignments, but is also valid for class C GPCR receptors, where no such pattern is observed. Jiang et al 79 have recently reported the successful identification of the critical determinants for the binding of an allosteric modulator, lactisole, for the taste receptor, T1R3, which is another class C GPCR receptor. This study is another experimentally validated example for a successful 7TM mapping of a class C GPCR to the rhodopsin template and is in agreement with the findings reported here for the metabotropic glutamate receptors.…”

Binding of glutamate to the umami receptor

“…Homology modeling of the closely related sweet taste receptors (T1R2 + T1R3) has facilitated an understanding of the interactions of sweeteners with their receptor [7][8][9][10][11][12].…”