Orphan sensory receptors are expressed in hepatocytes

Abstract

Sensory receptors, including olfactory receptors (ORs), taste receptors (TAS), and orphan G‐protein coupled receptors (GPRs) have recently been found in a variety of non‐sensory tissues where they have important physiological functions. As G protein‐coupled receptors (GPCRs), these proteins serve as chemosensors by interpreting and responding to chemical cues in their native environment (i.e. blood, urine) and can be activated by naturally occurring metabolites. We reasoned that the liver, the largest metabolic organ in the body, is primed to take advantage of some of these sensory receptors in order to regulate its internal environment. To identify the complete complement of sensory receptors expressed in murine liver, we performed an unbiased screen of whole liver using TaqMan qPCR array cards. This screen identified a total of 17 ORs, 10 TAS, 3 opsins, and 57 GPRs – with 7 of the GPRs expressed at levels equal to well‐characterized hepatic GPCRs such as glucagon receptor. In order to understand the functions of these sensory receptors, it is imperative to determine where they localize and their ligand profile. Due to the lack of reliable antibodies, we turned to RNAScope to localize mRNA transcript expression within the liver. Using this technique, we were able to confirm that murine Tas2r108, Olfr57, and Gpr137 are all expressed in hepatocytes, the metabolically active epithelial cells of the liver. With confirmation of expression, all three of these hepatic receptors were cloned from both male and female livers and expressed into heterologous cells for ligand screening. While Tas2r108 has known ligands – bitter tastants that have anti‐bacterial functions – both Olfr57 and Gpr137 remain orphan receptors. This is due, in part, to the inability to achieve functional expression of these receptors in vitro. Of note, Gpr137 has multiple splice variants and co‐expression of the canonical full‐length GPCR form along with a truncated variant increases surface expression of this 7‐transmembrane domain receptor. Ongoing studies are aimed at determining the role that Gpr137 plays in hepatic protein trafficking. Surface expression conditions were also achieved for Olfr57 with the assistance of a cleavable LUCY tag allowing for large‐scale ligand screening using a cAMP‐driven dual luciferase reporter assay. Using this platform, we determined that Olfr57 responds to terpenes including camphor and fenchol. These volatile organic compounds are known fungal metabolites, opening up the possibility that the liver is utilizing ORs to detect fungal growth. Efforts are currently underway to test this hypothesis and to establish the functional relevance for these chemosensors in hepatocytes.Support or Funding InformationNIDDK