Characterizing nutrient-activation of orphan and lipid metabolism-related G protein-coupled receptors

Abstract

Nutrients and food additives serve as energy sources or non-nutritive flavour enhancers for organisms. Advances in nutrient-sensing mechanisms and the discovery of the reciprocal relationship between metabolism and intracellular pathways have identified some nutritive and non-nutritive food components, namely amino acids (AAs) and non-nutritive sweeteners (NNSs), as ligands for G protein-coupled receptors (GPCRs). For example, AAs act as ligands for mGluRs, multiple class C GPCRs and the class A orphans GPR142 and GPR139. Meanwhile, NNSs of markedly different structures have been shown to activate the sweet taste receptors T1R2/T1R3. The consumption of AAs aids in managing food intake and glucose homeostasis, although the mechanisms underlying these metabolic benefits remain unclear. However, a recent study identified an intestinal trafficking mechanism of individual AAs leading to the activation of an AA-sensing GPCR, suggesting that GPCRs play a larger role in nutrient signalling than once thought. Food products containing NNSs have become increasingly popular over the past few decades to provide powerful sweetness, in part through the activation of the sweet taste receptors T1R2/T1R3, without the added calories. Despite being considered metabolically inert, the consumption of NNSs has been linked to an increased risk of obesity, type 2 diabetes, and metabolic syndrome with suggested off-target signalling through GPCRs independent of T1R2/T1R3. Ligand identification is critical in understanding GPCR function; as a result, we used a high throughput approach quantifying GPCR activation to identify novel nutrient-GPCR interactions. This approach was applied to two experimental questions 1) to what extent do AAs activate orphan and lipid metabolism-related GPCRs, and 2) do NNSs elicit off-target signalling events through interactions with GPCRs? Both projects primarily measured b-arrestin recruitment and used a panel of in vitro luciferase signalling reporters. Herein, we discovered that the AA L-phenylalanine (Phe) is a promiscuous endogenous ligand, significantly activating 148 class A GPCRs. Additionally, we found that the NNS sucralose increases the activity of the constitutively active orphan GPCR GPR52. These observations are the first to identify the widespread binding capacity of Phe through GPCRs and a GPCR other than the taste receptors as a target for sucralose. This study provides evidence of the interplay between metabolite sensing and the GPCR signalling network and may potentially inform pharmacological approaches to predict novel agonists or off-target effects.